2.5" HDD is normally known as notebook/laptop HDD... It's
smaller and more portable, no need to lag around an external power
supply... Comes in IDE and SATA...
3.5" HDD is heavy and bulky, normally known as desktop HDD... It
normally requires external power supply even if it's plug into the
interface of your computer or laptop... Also comes in IDE, PATA and
SATA...
USB 2.0 can transfer data up to 480 Mbps and Firewire is up to 400
Mbps... For convenience, some come with both USB and Firewire...
But not all computers and laptops have Firewire ports...
IDE is the older technology and SATA is newer and faster...