For a witch, an invocations is similar to another religion's prayers, with an added dimension in that we are not merely calling to the Deity, but calling forth the Deity from within.  Invocations are generally used during ritual, but also may be spoken (chanted, sung, thought, etc.) at any appropriate time.  Invocations are one of the ways we remember our link to the Divine, to the Lord and Lady, and feel the power of that linkage.