Ditto is just a novelty Pokemon. Its stats are horrible. If you want to try it, do it in your UU teams.
Ditto's only move is Transform, which makes it unique since Mew is the only other Pokemon that can learn it in the RBY generation. If you don't know what Transform does, it allows you to change into the defending Pokemon. You are able to use its moves (they only have 5 PP each) and you get all of the defending Pokemon's stats except for HP. However, since Ditto's Speed is rather bad, it usually takes a hit before Transforming, and with that bad HP stat, it may not be able to pull anything off.
Transform is unaffected by accuracy and copies stat-ups, so it can be useful for beating evasion-move users, but unfortunately it only ends up with 5 PP on each move. However, if you can switch out and switch back in, you will get 5 more PP of each move, so in a situation where someone is evading and not doing a lot of damage, Ditto could be useful then.
That's about it for Ditto though... Maybe if you start PP stalling an Amnesia Slowbro, and it Rests while you send Ditto, you could steal their Amnesias at a point where you have more PP than your opponent and then sweep. There are lots of extremely circumstantial situations like that, but unfortunately Ditto is not of a great deal of use generally.
Also, it is interesting to note that Ditto keeps its critical hit rate after it uses Transform. Furthermore, critical hits use a Pokemon's original stats, so any Attack, Defense, and Special changes due to Transform are ignored, which is kind of bad for Ditto.
Checks and Counters
Anything that's in the OU metagame and for the most part anything in the UU metagame will beat Ditto. It really can't take a hit with 48 base stats across the board.