PSALM 119 is sometimes called the Mount Everest of psalms. 

Not only is it long, its length is due to its unique construction: It’s an extended alphabet acrostic divided into 22 stanzas, one for each of the letters of the Hebrew alphabet. Further, each of the eight verses in every stanza begins with that Hebrew letter. For example, the eight verses of the first stanza all begin with alef, the verses of the second stanza with bet, and so on. 

The overarching theme of the psalm is a contrast between following the ways of Yahweh and the way of wickedness, with elements of a lament as the author, possibly David, cries out for God’s help in the face of persecution.