Intro to Laptop Hard Drives

Interface (connectors):

laptop PATA vs SATA http://www.laptopparts101.com/wp-content/uploads/2008/12/sata-ide-laptop-hard-drive.jpg

• Parallel ATA a.k.a. IDE, ATA, ATAPI, UDMA and PATA — legacy, wide 40-pin connector for disks produced few years ago. In case of notebook drives pins are smaller, and there is also power supply in the same plug.
  

• Serial ATA (SATA) — modern connector. Most modern laptops use it. 6 data pins and 15 pins including power supply.

Sizes of laptop disks:

• 2.5" — most common
• 1.8" — reduced, used mostly in ultralights and netbooks.

Rotation speed:

• 7200RPM — modern, high-end 2.5" disks. Consume more energy.
• 5400RPM — standard, low-end 2.5"disks or high-end 1.8"disks. More energy-efficient.
• 4200RPM — legacy, low-end 2.5" disks, some modern reduced height and energy-efficient 2,5" or standard 1.8"disks.

Take in account, that 5400RPM HDD with bigger capacity actually might have faster transfer rates, than smaller capacity 7200RPM. Rotation speed does however directly affect seek times.

HDD vs. SSD:

Modern alternative to mechanical HDDs are Solid State Drives, based on flash memory. They have almost instantaneous seek times, incredible read speeds and very low power consumption. As of now they are still much lower capacity than similarly priced HDDs. However, with the effects of Thailand floods and sharp drop in SSD prices in recent years, there are no longer excessively expensive. SSD can come in 2.5" SATA form factor, thus be interchangeable with 2.5" HDD. Another form factor, unique to SSDs is Mini-SATA (mSATA), intended mostly for use with netbooks (and some ultraportables). Below mSATA drive on top of 2.5" SATA HDD for size comparison:

Note, that ultrabooks use neither of these formats. In ultrabooks SSDs are soldered permanently onto motherboard, thus cannot be removed nor upgraded.

You asked how to shop for notebook hard drives. I prefer to hit my favorite review sites and look in their storage review sections. One of these always seems to have a good comparison review between hard drive vendors:

• TomsHardware storage section - most recent comparison is the 2.5" comparison charts. These are way too detailed, though.
• Anandtech storage section - good for SSD reviews

The one thing to be aware of is that there's some high-capacity drives that are taller than other drives. Your laptop may only tolerate a 9.5mm high drive, for example - so don't go buying a 12mm high drive unless you know it'll fit.

There are, really, only two types of drive connections: Regular old IDE (PATA) and SATA. Each use their own connector, and more importantly data transport protocol. So you have to buy the correct interface for your laptop. That being said most laptops will have another, specialized, connector that actually hooks the drive into the controller. As a result it can, on first blush, be non-obvious whether you are working with a SATA or PATA drive. That connector, however, should come off the old drive easily. Once you've figured out whether you need IDE or SATA look for 4 things. Size, Speed (rotational and seek time), Cache, and Warranty. First, figure out the size that you want, then try to maximize the rest based on the price you're willing to spend.