Psalms - Chapter 23
Possibly the Most Famous Psalm of All
(Seriously. Google Psalm 23 and you'll get almost 2 and a half million returns.* Psalms 22 and 24 each get one-third that result.)
This Psalm was composed by David at one of the most perilous junctures of his life, while he was fleeing from Saul. This heartfelt Psalm really speaks to people, no doubt contributing to its familiarity. It's so familiar, it almost seems silly to paraphrase it here.
David says that G-d is His shepherd, taking care of all his needs. ("I shall not want.") Continuing the metaphor of a sheep, G-d gives David green pastures and calm, sweet waters. G-d refreshes David's spirit and leads him on the proper path. Even though he walks through perilous places, David need not fear with G-d in control. ("Yea, though I walk through the valley of the shadow of death, I will fear no evil...") G-d's "rod" and "staff" (referring to punishments and mercies) comfort and support David. (The Radak notes that the rod and the staff refer to the same object; the difference is in how it's used at the time.)
Leaving the sheep metaphor, David says that G-d has prepared a table for him, so that his enemies can see how G-d takes such good care of him. He anoints David and fills his cup to overflowing. David hopes that he may always enjoy such goodness and that he may serve G-d in peace for a long time to come.
This Psalm is typically recited at Shalosh Seudos (Seudah Shlishit, the third Shabbos meal).
*I wrote this a long time ago! It's more like 56 million now!