The builds of Android (i.e. ROMs) are specific to a make/model of handset or device (tablet, media player, etc). For example, you cannot run a ROM that is meant for the HTC Magic on the HTC Dream as they have different hardware.
For official ROMs it gets more complicated, as usually the mobile network will customise the ROM slightly (branding, logos, ringtones, apps, etc.) for their own purposes. This means that a phone purchased from a specific network will only accept ROMs that have been updated by that network. However, there are ways around this if you want to root your phone and then install a custom ROM.
Check out djunio's link above to Modaco - from there you'll be able to root your phone and try out the custom ROMs.