methodology differences

These are all different flavors of lite methodologies. IT Republic has a number of articles on these you can download and read. Alternatively, I would put the names into a search engine such as Jeeves or Google - you should get plenty of hits. I am most familiar with RAD - rapid application development, which is a bottom up approach to project development; creating an 80% solution by prototyping the product through several iterations for user signoff and then completing the background coding.

You want basic, so here goes ...

OO - Some academic stuff.
UML - Some abstract stuff.
RAD - Some action stuff.

It is a 4-year-course by itself understanding these methodologies. It is never wrong to say they are all the same and yet different. But if you are only a user, try this :

Use OO to design meeting your existing needs, then use RAD to test your designs. You should have many rounds of failures. Once you got what you want, use UML to predict what you might need and generalize your product.