More from Prof. Pless, drawing on the writings of Australian theologian John Kleinig. . . .When you thank someone, you address that person directly. When you praise someone, you tell someone else how great that person is.
When it comes to God, we are to thank and praise Him. We tell God how grateful we are to him–for His grace, His blessings, our existence, and on and on–in thanksgiving. Praise, strictly speaking, is when we tell other people how great He is, what His wonderful attributes are, how good and gracious He is.
When the Psalm says “Praise Him,” we should do so in talking about Him to others. Thus, hymns of praise are for the benefit of the singers and the hearers and their lyrics should thus have substantive content about who God is and what He has done.