Passport is the 2.5" Western Digital drive. When choosing a drive this is what you need to consider... There are two sizes and 3 different types of connection:
2.5" Drive
These are the same drives as used in laptops, at the moment they go up to about 500gb in storage and generally spin slightly slower than desktop drives. Because of the small size and slower spin they use less electricity and generate less heat, this allows them to be powered purely by the computer via the USB lead. So these are ideal for carrying with a laptop or for frequent chopping and changing. Performance is generally slower than 3.5" drives.
3.5" Drive
These are the same drives used in desktop computers, they can go up to about 1.5tb (1500gb) and often run at the higher spin speed of 7200rpm. They generate more noise, heat and always require a separate power supply to power them. More often used for backup devices due to their larger capacity.
Connection Types:
USB - Most common connection and can be found on all computers now (for the last 7 years or so). This currently runs at around 400mb per second on newer machines (5 years or older you may only get 200mb) This is normally find for backups and day to day use. Only requires a power plug for larger drives.
Firewire - Commonly found on Mac's but also present on some laptops and PC's, especially used for video editing. Can run at either 400 or 800 mbps (depending on plug) and does not rely on the computer processor so is generally quicker than USB. Downside is that not many machines have it, not usually used unless you really need that boost in speed. Almost always requires a separate power plug.
eSATA - New interface just starting to appear in PC's and some laptops. This is an external version of the connector found inside most PC's to connect disks and as such is much faster than either firewire or USB. The cable is usually stiff and unweildy compared to USB / Firewire. Usually has a price premium and unless you have a specific need to have the extra speed is probably not worth consideing at the moment. Always requires a power plug.