Preprint format gives double-spaced, full page output. This is useful for drafts, and this is the format that the APS likes to have its papers submitted. Figures and tables go at the end of the document.
Tightened Preprint is like preprint, but the lines are single spaced. This is an appropriate format for group notes etc.. This document is in Tightened Preprint format.
Twocolumn format gives an output which looks very much like a journal
page. This is useful to make a final version of a paper to post on
the web page and to distribute as preprints. The output is pretty
spectacular. In this case the figures are embedded in the text close
to where they are referenced using encapsulated postscript and some
style files for handling this in LaTeX. The optional argument
to \documentstyle which
gives this feature is ``floats'' and there are some other LaTeX commands
which begin \eps... .
Floats can be used in preprint format also. It is appropriate for a finished document such as a preprint for distribution or a group note, but not a draft paper being prepared or submitted. The best approach is to write the paper in untighted preprint format then, once it is submitted, make a second version with embedded figures and perhaps twocolumn format. It is a small amount of work becuase only a few commands have to be changed.