notebook hard disks come in two sizes: 2.5" and 1.8". Most users will never have to worry about 1.8" drives; these never appear in more consumer-oriented retail stores. They're only used in ultraportable notebooks (usually Fujitsu, Sony or ThinkPad ultraportables) and are almost always not upgradeable.

2.5" hard disks are what just about everyone uses in their notebook. These have just recently been showing up in numbers in retail chains, likely as a result of the skyrocketing popularity of notebooks in general, and are commonly referred to on packaging just as "notebook hard drives."

notebook drives come in IDE and Serial ATA(SATA)

Many of notebooks today use conventional IDE (or ATA-5/ATA-6 in notebooks) as a connector, and that's what almost all (if not all) the notebook hard disks on the shelf at the local retail store are using.

Identifying which kind of connector your notebook uses (and thus, which type of hard disk to get) is fairly simple. If the connector on the back of the drive is a two row grid of pinholes about two-thirds the width of the drive itself, it's IDE. If not, it's Serial ATA.

Notebook drives come in three different speeds: 4200rpm, 5400rpm, and 7200rpm. These speeds refer to the speed at which the spindle inside the drive rotates. Without going into too much detail, they basically refer mostly to how fast data can be accessed from the drive.

Now, it stands to reason that the faster a drive spins, the more power it's going to eat. By and large that's true, but it's not the great difference anyone thinks it is. In fact, some 5400rpm drives report lower power usage than 4200s. If anything, the power consumption of a 5400rpm drive is about on par with a 4200. The rare 7200rpm drives do eat a little more power, usually cutting your battery life by a few minutes.

Another major consideration associated with drive speed is heat. 4200rpm and 5400rpm drives will run much cooler than a 7200rpm drive will, and if your notebook already has heat problems (like mine), the move to a 7200rpm drive won't be a good choice.

The faster a drive is, the more expensive it's going to be. There's a decent price hike from 4200rpm to 5400rpm and then a massive one from 5400rpm to 7200rpm.

Another, "new" type of a mobile hard drive is a SSD(Solid State Drive)

A solid state drive (SSD) is a data storage device that uses solid-state memory to store persistent data. An SSD emulates a conventional hard disk drive, thus easily replacing it in any application.

With no moving parts, a solid state drive largely eliminates seek time, latency and other electro-mechanical delays and failures associated with a conventional hard disk drive.

SSDs based on volatile memory such as SDRAM are categorized by fast data access, less than 0.01 milliseconds (over 250 times faster than the fastest hard drives in 2004) and are used primarily to accelerate applications that would otherwise be held back by the latency of disk drives.

Solid state drives are especially useful on a computer which already has the maximum amount of RAM. For example, some x86 architectures have a 4 GB limit, but this can effectively be extended by putting the paging file or swap file on an SSD. These SSDs do not provide as fast storage as main RAM because of the bandwidth bottleneck of the bus they connect to, but would still provide a performance increase over placing the swap file on a traditional hard disk drive.

Advantages:

Faster startup (as no spin-up is required).
Faster random I/O (compared to conventional disks).
Extremely low read and write latency (seek) times, roughly 5 orders of magnitude faster than the best current mechanical disks.
Faster boot and application launch time when hard disk seeks are the limiting factor. See Amdahl's law.
In some cases, somewhat longer lifetime – Flash storage typically has a data lifetime on the order of 10 years before degradation. If data is periodically refreshed, it can store data indefinitely.
Few to no mechanical parts.
Security – allowing a very quick "wipe" of all data stored.
Relatively deterministic performance
For very low-capacity drives, lower weight and size.

Disadvantages:

Price – As of early 2007, flash memory prices are still considerably higher per gigabyte than those of comparable conventional hard drives – around US$8 per GB compared to about US$0.25 for mechanical drives.
Vulnerability to certain types of effects, including abrupt power loss (especially DRAM based SSDs), magnetic fields and electric/static charges compared to normal HDDs (which store the data inside a Faraday cage).
Limited write cycles.
Slow random write speeds
Speed advantage of SSDs can be overcome by RAID setups of conventional HDD, which may have more storage and speed for a much lower cost.
In some cases, SSDs have substantially lower throughput than conventional hard disks. In spite of the decreased latency, this can lead to dramatically lower performance than conventional drives. More expensive SSDs can have much greater bandwidth than conventional hard disks, so this isn't universally a problem.