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.

I have many invocations in my Book of Shadows... here are some for you to contemplate:

Goddess Invocations
Artemis -- four
Chant  by Medea
Crone
Diana
Full Moon
Goddess -- several
Hecate
Ishtar -- two
Isis
Lady -- two versions of the 'Lady's Prayer'
Triple Goddess Offering

God Invocations
God
Hermes
Lord of the Dance
Pan

Both
A Call to the Lord and Lady

Miscellaneous Invocations
Angelic
Devotional
Earth and Sky Mothers
Earth Invocation from Homer
Lamp of Wisdom
Meal Devotional
Morning
Night
Nightway -- from the Navaho
Setting Sun
Two Ritual Invocations