I haven't used UG since version 15 I think, but what I remember is that the two are similar in some respects. They both use sketches to define shafts and pads. One of the biggest differences you are going to find is the terminology and icon differences from one software to the next. I am sure that some of the methodology is going to be slightly different also.
Catia is set up to have a master model or part (catpart) and then linked files for drawings (catdrawings). Each part (catpart), is meant to have only 1 body, assemblies are to be done in a assembly model (catproduct). But what I remember about UG, it was recommended that you use a master model, but you were able to create drawings in the same file as the part was in.
Another thing, if you are used to being able to do radial sectioning in drafting, you aren't able to do that in Catia V5.
Good Luck!