"While many of us can create something that looks good in Photoshop or attractive when spliced into CSS, but do we actually understand the design theory behind what we create? Theory is the missing link for many un-trained but otherwise talented designers. Here are 50 excellent graphic design theory lessons to help you understand the ‘Whys’, not just the ‘Hows’."
this tells you everything you need to know and worth a read, from typography,grids and colour and alot more info.
This is very useful site that can help me have a better understanding of graphic design theory that i may not have grasped and hopeful can help improve in my designs from this as a result.