According to Dillard & Longman (p. 220 Introduction to the Old Testament) in a lament the psalmist "experiences trouble and turns to the LORD for aid". That trouble comes from within or without, even God himself (Ps. 102:9-10). It is a song of "disorientation, of abandonment, distress, pain, and suffering."
The form of the lament has a distinctive structure consisting of more than one of the following elements:
1) Invocation
2) Plea to God for help
3) Complaints
4) Confession of sin or an assertion of innocence
5) Curse of enemies (imprecation)
6) Confidence in God's response
7) Hymn or blessing