BIG difference.
Absentee voting is just what it says...you won't be in your precinct when the election takes place so you request an absentee ballot and cast your ballot.
Absentee voting has been around for DECADES and is generally speaking trusted by everyone, no matter what party affliation.
Mail in ballots are just sent out, willy-nilly, to every residential mail box in a given area, anyone can fill it out, there are no checks on exactly who cast the vote or who turned the vote in, and if it is received after the election, it is still counted.
Mail in ballots are relatively new, subject to massive fraud, and only trusted by one party...the one that controls the US Postal Service and its UNION employees.
Like I said...BIG DIFFERENCE!