hard drive info

Hi,
I am thinking about using a external H/D for back up, running 40GB IDE seagate
barracouta 7200rpm at the moment.
Problem is that looking at H/D's it states 2.5" and 3.5" see below:

Seagate External 2.5" 40GB Usb2 External Hard Disk Drive 5400RPM 2MB
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Seagate External 250GB USB 2.0 3.5" Hard Disk Drive 16MB Buffer
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Don't realy know what that is about and don't want to get H/D and then find it is no good.

Can some one please tell what 2.5" and 3.5" means ??
Many thanks.
Espinoso.