Commands:

• \uncover<n-m>{}: uncover content only for slides n-m (syntax “n-” or ”-m” allowed)
• \only<n-m>{}: same as \uncover, except it doesn't reserve space for slides other than n-m
• \alt<n-m>{}{}: uncover first argument for slides n-m, and second argument else

Environments:

• textblock in order to put content at any given place:

  \usepackage[absolute,showboxes,overlay]{textpos}
  %\TPshowboxestrue % for development, comment when done
  \TPshowboxesfalse % uncomment to hide boxes
  \textblockorigin{10mm}{20mm} % positions origin

  \begin{textblock*}{110mm}[0,0](0mm,20mm)
  \end{textblock*}