What is the difference between GO terms and GOSlim GOA?

GO term usually refers to the original full set of annotations maintained by GO consortium. GOSlim refers to the cut-down (slim) versions of the GO ontologies containing a subset of the terms in the whole GO. GO term based annotations will provide a complete view of functions / process / compartments annotated to your gene lists. Where as GOSlim will provide a concise view.

When I do the gene ontology analysis, which one I should use?

Both are useful for functional interpretation. There is no defined criteria for selecting the annotation database as GO or GOSlim. I would recommend you to try your analysis using both and see which provides you a more compact view of the functions / process / compartments related to your gene lists.