Finish : the effect of the paint or coat
EG gloss, Matte or satin (in between gloss and matte)
Enamel Thinner : it thins the paint!! you usually have to do this for airbrushing, otherwise the paint splutters out, and thats not what you want.
Primer : Its a coat of special paint which helps other paints grip onto it
I dont use primer, but you use it before the first coat of paint goes on
Enamel thinners are mixed with the paint to go on ( Dont use Too Much!)
Finish, put over the paint, at the end
Hope this helps!