Hi ya fluffy,
Buckskin is the dilute version of bay ie: horse carries the cream gene which acts to dilute the 'red' but not the black in the coat hence creamy/gold colour with black points. A black horse may carry the cream gene but it will not show. This is shown as Ccr to describe the presence of the cream gene. It is to bay what palomino is to a chestnut horse.
Dun is another gene again and is expressed D. if your horse carries the dun gene it will always show two characteristics. The gene acts to dilute both red and black pigments in the coat - and the horse will display two or more of the 'dun factors' which are primitive markings consisting of dorsal stripe, leg barring, shoulder barring, cobwebbing on face, colouring to ear tips - Christine help me out if I have missed any here.
Not every horse that has a dorsal stripe is a dun though, so look for evidence of the other characteristics. Take a look at Whitewood Quarterhorses website for examples of all different types of dun - my boy is on there too ;D
If you want a more technical explanation try:
http://www.vgl.ucdavis.edu/~lvmillon/coatcolor/coatclr3.html#fig2aFor an informative read on equine coat colour.
Cheers, Jude.