wondering the difference between some similiar drug classifications pharm

If someone could help me to understand the difference between some drug classisfications that seem similair to me. Im going to try to find out from my teacher but in the mean time I thought someone may be able to help. Difference between Histamine h2 antagonists and antacids, difference between Salicylates and nonsteroidal antiinflammatories. Also beta blockers and ace inhibitors. I kinda know what cholinergic and adrenergic are but what would be some major things they do or are used for. I already have looked in my drug book and the internet. Thanks