Capacity

Hard drives come in various sizes/capacities. If you plan on storing a lot of photos, videos, music, etc. on your computer or installing lots of extra programs, you should look in to getting a laptop/tablet with a larger hard drive. If you won't be storing many of those larger files on your computer and will mostly just be storing Word documents, you may be able to make do with a smaller hard drive. The capacity of a hard drive is most commonly measured in gigabytes (GB) or terabytes (TB). There are 1024GB in 1TB.

Traditional HDD/SSD/Hybrid

A traditional hard disk drive (HDD) is composed of moving, magnetic disks that store data. It is susceptible to physical failure which can result in the loss of files/data. HDDs can store a very large amount of data and are fairly inexpensive compared to SSDs.

A solid state drive (SSD) contains no moving parts which makes it less susceptible to physical failure as well as being a little quieter. It also is able to access data more quickly which can result in your computer starting up and or running a little faster. SSDs are a newer technology and are still fairly expensive compared to the traditional HDDs. Because of this you won't usually find computers that come with very large capacity SSDs. 

Hybrid drives are somewhat like traditional HDDs that have a little bit of SSD added to it. Because the technology from both HDDs and SSDs are combined, these hard drives can allow a computer to start up faster and maybe run a few programs a little bit faster. Hybrid drives aren't quite as fast as SSDs, but they are faster and only a little more expensive than traditional HDDs. Because hybrid drives use HDDs, they are also susceptible to physical failure.

Speed

Hard drives can have different drive speeds. These drive speeds can affect how quickly a computer can save/access files stored on the hard drive. Greater drive speeds mean that the computer will save/access the files more quickly. The speed of a HDD is measured in revolutions per minute (rpm).