I would like some advice about replacing wood windows.

a) Is this a relatively easy DIY job? Seems so, but are their gotcha's
b) Vinyl or Wood? I think I like the idea of vinly, but are their
advantages of wood (cost, etc).
c) Can anyone recommend where I can purchase replacement windows in the West
Los Angeles Area..unless there are online sites that work out better.
d) Any good online resources for these questions?

Thanks In Advance