Oddly enough, back in the day, I saw the series out of order. I saw 2, "Army of Darkness" then The Evil Dead. And apparently it was ok, since 2 is basically just a remake of the first.
In the first, five friends drive up to a secluded cabin for a weekend. They stumble upon a tape recorder that has some evil spells on it, which awakens the evil dead. Soon, most of the people are possessed and the dead arise and raise some hell. Soon, Ash (Bruce Friggin Awesome Campbell) is the only one left as he battles the evil on his own.
In Part 2, Ash and his girlfriend go to the same cabin and almost immedately, everything from the first happens, including his girlfriend getting possessed and dying within the first ten minutes. The rest of the movie Ash battles with the evil, which takes control of his right hand, so he's forced to cut it off. Eventually, a group shows up, one of which is looking for her father. They believe Ash killed him but after playing back the tape recorder they learn the truth and soon everyone is possessed. The daughter manages to open a worm hole to send the evil back but Ash (and his car) get sucked into it and they land in the 1300's.
Army of Darkness picks right up there, with the "primative screwheads" thinking Ash works for an enemy camp so they throw him into a pit full of undead demons. Ash proves he's a badass by taking care of the pit. Soon, he asks for a way home and the wiseman says he needs to get the book of the dead in order to send him home. Ash goes on a journey, meets his evil twin, and he gets the book. But since he didn't say the words right, the army of the dead arise and want the book back, so it's up to Ash to teach the screwheads how to fight them with medevil armor and some 20th century technology.
Progressively, each movie got goofier and goofier as it goes on, with Army of Darkness full of many one liners. So I'll break it down like this:
If you want a real creepy scary ass movie, get The Evil Dead.
If you want a mildly creepy movie with some laughs, get Evil Dead 2.
If you just wanna hear all the great Bruce Campbell one liners, get Army of Darkness.