If LaTeX responds to a \begin{figure} or
\begin{table} command with the error message
! LaTeX Error: Too many unprocessed floats. See the LaTeX manual or LaTeX Companion for explanation.your figures (or tables) are failing to be placed properly. LaTeX has a limited amount of storage for 'floats' (figures, tables, or floats you've defined yourself with the float package); if you don't let it ever actually typeset any floats, it will run out of space.
This failure usually occurs in extreme cases of floats moving "wrongly"; LaTeX has found it can't place a float, and floats of the same type have piled up behind it. LaTeX's idea is to ensure that caption numbers are sequential in the document: the caption number is allocated when the figure (or whatever) is created, and can't be changed, so that placement out of order would mean figure numbers appearing out of order in the document (and in the list of figures, or whatever). So a simple failure to place a figure means that no subsequent figure can be placed; and hence (eventually) the error.
Techniques for solving the problem are discussed in the floats question already referenced.
The error also occurs in a of fooling LaTeX that you're inside it
long sequence of figure or table
environments, with no intervening
text. Unless the environments will fit "here" (and you've allowed
them to go "here"), there will never be a page break, and so there
will never be an opportunity for LaTeX to reconsider placement.
(Of course, the floats can't all fit "here" if the sequence is
sufficiently prolonged: once the page fills, LaTeX won't place any
more floats, leading to the error.
Techniques for resolution may involve redefining the floats using the
float package's [H] float qualifier, but you are unlikely
to get away without using \clearpage from time to time.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=tmupfl