I wrote this for a friend and then decided to post it here. If you would like me to expand on something or clarify, please comment.
The top laptop brand, in my opinion, is Apple. Apple hardware is more reliable and better designed, as is their software. This comes at a price. It is up to you to decide whether it is worth it. I have an iBook G3 from 2001. I still use it to watch DVDs, and the battery still lasts through an entire film.
However, if we limit the discussion to PCs, it is probably best to start with the hardware in question, as it is this that informs our comparison of brands.
To jump to features – generally, when we are comparing laptops, we are concerned with the hardware. The software is usually the same, or can be made so. There are several factors, the relative importance of which, depends on your use case.
The hard drive
Two factors here: space and speed. Space, obviously, is the measure of how much data it will contain. If you are storing a ton of photos, movies, or anything else which has a large unit size, the space can be a very important factor. Standard today is around 500 GB, large is 750GB or even 1TB (1024GB, see below), and small is usually about 256GB.
Sidenote regarding space units. One gigabyte is equivalent to 1024 megabytes. One megabyte is equivalent to 1024 kilobytes, etc… The reason for the usage of 1024 rather than 1000 comes down to how a computer stores and computes data. The usage of binary means that data size will generally be expressed in binary units, e.g. 210. However, hard drive manufacturers will express hard drive space in terms of 1000, the result being that you can buy a 500GB hard drive and it will show up in Windows as a 456.7GB hard drive. Quite silly. Macs solve the problem by also computing space in terms of 1000, for better or worse.
The other factor with hard drives is speed, usually expressed in RPMs (revolutions per minute). Hard drives have platters that spin and are read by a sensor on a stick called a read head. The faster they spin, the faster you can read data, but the more power they use. So a higher rpm rating means better speed and less battery life. Laptops usually come with a 5400 RPM hard drive, but I usually try to find a 7200 RPM hard drive, as it helps performance. High end hard drives can be 10,000 to 15,000 RPMs, while low end or low power hard drives can be 4800 RPMs. The speed will help will boot time, starting programs, and copying or moving data.
There is a newer storage tech called a SSD (solid state storage). This is essentially a really big, really fast flash drive, like you find in a camera. SSD performance can vary widely, from extremely slow, to faster than most hard drives. They have very good battery life, as there are no moving parts. Speed is usually measured in MBs per second – that is, how much data can be transferred or written in a second. This measurement is also useful to compare SSDs with traditional hard drives, although random vs. sequential data access becomes an issue (see below). The biggest problem with SSDs is the cost per space is very poor. I can get a 60 GB SSD for the cost of a 500 GB hard drive.
Sidenote regarding random vs. sequential data access. When a file is read from a hard drive, the speed in reading it is very dependent on how the data is spread out over the platter. If all of the data is in a row, the read head only has to spin the platter once, and it has read all of it. If the data is spread out in a bunch of different places on the platter, the data is slower to read, as the read head has to move all over the place. This is a random read. SSDs don’t have this problem, as a random read is just as fast as a sequential read. Thus, one program may make a hard drive seem faster than a SSD, while the same hardware can yield the opposite result with a different program/workload. So much for hard drives.
Memory
Again, there are two factors, size and speed. However, this is simpler as you will only notice the difference between memory speeds in specialized workloads. So we only have to worry about memory size. Memory size determines the number of programs you can reasonably run at the same time, the amount of data you can handle in any given program, and how fast your hard drive appears. Memory is known as volatile storage. It is much faster than a hard drive or SSD, however when you unplug your computer, everything on it is lost. It requires power to store the data. Files and programs that are actively used are pulled from the hard drive to memory and the written back to the hard drive when you are done. Memory can also be used to speed up hard drive reads by caching frequently used files, and hard drive writes by holding the file in memory and writing the the hard drive in the background while the program thinks it has already been written. (this is, incidentally, why you can lose data if you unplug your computer without shutting down, or pull put a USB key without safely removing it.) This goes the other way as well: if a computer is running out of memory, it will start grabbing sections that have not been accessed in a while and write them to “virtual memory” on the hard drive to free up that section of memory for something else. This is called swapping and slows down your computer astronomically. A standard size for memory is about 4G, although 6 and 8G computers are available now. You can get 1, 2, and 3 G computers, but these cannot do as much. Memory is probably the easiest part on a laptop to upgrade, and replacements are readily available.
CPU
This is really complicated and changes frequently. There are several primary factors and host of smaller ones. The big factors are: number of cores, clock speed (gigahertz), cache, word size (e.g. 32 or 64 bits), and power usage.
I would try to have at least two cores. This increases system stability because one rogue process cannot take down the system. It also increases speed when multitasking. If you can get three or four cores, still better.
Clock speed is less of an issue than formerly. It makes a difference, but most people won’t notice compared to other factors. Try to not go below 2.5Ghz.
Always get a 64 bit processor. 32 bit is really old and is being phased out.
Power usage requires some research and can be inversely proportional to speed. However, some processors are better this way than others.
If you are looking for brands, there are two main ones: Intel and AMD. I am going to detail Intel as I am not very familiar with AMD’s models anymore. In Intel-land, I suggest going for a Core i3, i5, or i7. Pentiums are decent, but outmoded. Celerons shouldn’t be touched, they are ancient and slow. The Atom processor is slow, but uses very little power, and you will usually see in netbooks.
Screen
The two factors are size and resolution. Laptop screens are normally 15″ (measured diagonally), 17″, or 14″. You can also get smaller screens, but they tend to be more specific purpose. This is a very personal decision. You have to evaluate ease of reading, size, and weight to decide what you want. (Sidenote, most MacBook Pros are sold 13″. For some strange reason, they are closer in usefulness to a 15″ PC screen. They also come in 15″ and 17″ varieties.)
Laptop resolution determines how many pixels are on the screen. This means how much stuff you can see at the same time, and how big the text is. Again, a personal decision.
Case
This is one of the biggest, yet most overlooked factors. The weight and solidity of the case is a huge determinate of how long you will use the laptop, and how much you will like it. Apple is king here, but there are some other good systems.
Peripherals
What ports are available? Where are the ports located? It is helpful to have at least one USB port on each side, and hopefully more. Other ports to keep in mind are: camera card readers, HDMI, VGA, eSATA, and audio. This list is not exhaustive.
Battery
The number of cells determine how long the battery will last on a charge. This is usually pretty well defined. Another, far harder to determine, factor is the number of recharge cycles, or how long will the lifetime of the battery be. Apple tends to be very good here as well.
Conclusion
Once all of these have been evaluated, I really like (in descending order): Apple, Lenovo, Acer, and HP. There are other brands of better and worse quality, and different laptops within a brand will be of better or worse quality. Hopefully, this will help you make a good decision.