The first word in Psalm 103 is “bless.” The Hebrew word is barak and in the Septuagint, the Old Testament in the Greek language, it is eulogeo. Some Bible versions render these words as “praise,” but there are other Hebrew and Greek words for praise (hallelu, hallei, and yadah in the Hebrew; and epainos in the Greek). While the definitions are similar in nature, I think they are distinct enough to call out the differences; especially when the first line in Psalm 103 is:
Bless the Lord, O my soul.
The question this raises is how do we bless the Lord?
The essence of the word ‘bless’ is a bit complex. The meaning, according to various references, is centralized in the interior of our soul. Praise, however, is more centralized in the external expression in a worship service or some other outward demonstration of faith.
So what’s the big deal?
I think the big deal is, as Bullock states, “The person who praises must endow the vocabulary of praise with content. We can praise God without using the special language of praise, but we cannot long maintain the genuineness of that language without relating His being and works”. To bless God, then, is a recognition of specific works of God with an intentional expression of thankfulness for those specific works.
For instance, Psalm 103 gives some very specific works that we can intentionally express gratitude for. He forgives our iniquities and heals our diseases (v. 3). He redeems our lives from destruction (v. 4, and my personal favorite). He crowns us with loving kindness and tender mercies and satisfies us with good things (vv. 4-5). He’s merciful and slow to anger (v. 8); and on and on it goes in just this psalm alone.
I think the lesson in Psalm 103, and others like it, is that blessing God’s heart is our expressing thankful acknowledgement for specific things God has done for us. Such specific things can be reciting what we read in Scripture, or, more personally, deliverances God has done specific to our own lives. For example, I was facing a significant challenge at work but with the click of an email from someone else, the challenge dissipated away, leaving me stunned and inexpressibly grateful for His deliverance. He brought deliverance in a way that was completely unexpected. Another example is if one of my sons says he loves me, that warms my heart; but if he states something specific, like the way I help him with his homework or the times we spend together in the wilderness or on our bikes, I’m deeply blessed. Why? Because he shared a specific thing that brings blessing to his soul which then in turn brings blessing to mine. Why would our heavenly Father be any different?
Praise on the other hand, while a good thing, is often motivated by some external stimuli such as music or maybe a touching play or movie. Such stimuli may spark momentary thankfulness in a more general sense, but it often fades away as the stimuli fades away. Blessing God, though, is internally motivated and, because it’s in the depth of our souls, is not reliant on external stimuli and can be recalled at any moment throughout our day.
So while the differences may seem minor, I still think they are distinctive enough to understand. Besides, if the distinctions were unimportant to God, why would He go to such great care to use such distinctive words in Scripture?
 C. Hassell Bullock, An Introduction to the Old Testament Poetic Books: Revised and Expanded, Moody Publishers, 1988; Kindle location 2850.