I think the only difference is that minicamp is mandatory while OTAs aren't.
Skins are in the middle of OTAs right now, we have ours before our minicamp June 16-18. OTAs are basically where the team gets broken up and reports to their coordinator for meetings (offensive and defensive). Then on the field, the players meet with their position coaches and do various speed and timing drills, practice and learn new plays, etc.
One example of a drill Gregg Williams uses with the defense is to put 3 cones at designated points on the field, then tell 4 guys to run after the cones, the guy that doesn't get the cone has to do extra pushups.
Its more a learning experience and building camraderie then anything else.