Short of watching the movie cuts side by side, it would be very hard to get more specific than the wiki article does.Yet ultimately, they can be summed up as CGI and continuity changes.
George Lucas Felt like the technology at the time limited his vision for what he wanted it to look like. Lots of changes are like that. Other changes are for story. The prequels had many characters that play the younger versions of the characters in the original trilogy, so some changes are simply actors from the prequels replacing the actors in the original. (for example: the voice of Boba Fett, Anakin force ghost in ROTJ, and the hologram of the emperor.)
Here are some videos of the comparisons because, like I said, it is the best way to see all of the changes.
New Hope changes part 1
New Hope changes part 2
Empire Strikes Back changes
Return of the Jedi changes
The Phantom Menace: Yoda changes, puppet to CGI
Another list I found just now that you might like