Ignorant question about full frame and APS-C

[dvxusr]

I apologize in advance but I see that lenses on some store sites are categorized into full frame and not. I assume mount doesnt change at all a Canon is a Canon. Are these interchangeable and if so what happens to the image if lets say at some point i got a full frame camera like a 5D and wanted to use lenses i had (old manual or newer AF for 7D) on it?

[ydgmdlu]

You get a much wider field-of-view when you go from APS-C to full frame with the same lenses.

Be aware that some lenses are made for APS-C only and won't work on full frame. You can mount the lenses, but you'll have an unusable amount of vignetting. So if you intend on moving to full frame later, do NOT buy lenses not made for full frame.