Custom GUI Elements in Inspector

I'm a little lost with some of the Custom Editor/GUI Stuff, and I'm probably getting lost in terminology rather than anything else.

I have a regular script that has various elements that are editable in the inspector. However I want to create a different way of entering the data that would be more suited to the task.

For instance a progress bar, or drop down that takes a string array or something.

How do I create (and then use, in a script) a custom GUI element in the inspector?