The Psalms is called the book of praises, sepher tehillim.  Tehillim is defined as “all that is worthy of praise and celebration.”  What is worthy of praise?  The answer is the pervading theme of the Psalms – the works and ways of the Lord (Jehovah).  Psalm 1 sets the pattern of emphasis throughout the whole book.

Psalm 1:1-3:
Blessed is the man that walketh not in the counsel of the ungodly, nor standeth in the way of sinners, nor sitteth in the seat of the scornful.
But his delight is in the law of the Lord; and in his law doth he meditate day and night.
And he shall be like a tree planted by the rivers of water, that bringeth forth his fruit in his season; his leaf also shall not wither; and whatsoever he doeth shall prosper.

The man who delights in the Lord and in His Word shall prosper.  The problem of the suffering or enduring of the righteous versus the prosperity of the wicked is set at the beginning.  The solution is that God always triumphs over all and when a man is cared for by the Lord, he shall prosper and bring forth much fruit.

The praises in the psalms show how God brings deliverance, peace, joy, health and protection to His people.  This may be in the form of a song (shir) or meditation (higgaion) or prayer.  It is invariably the expression of the magnitude and depth of God working with His people.  The thematic content of the Psalms is in essence, personal.  It is an I-thee relationship where the believer is describing, imploring, worshipping, magnifying God.  The “I” may mean an individual or apply to a whole congregation.  It is still basically an individual expression.  This personal content means that the psalm is written out of a particular believer’s heart and life.  It may come out an historical event or situation or out of a moment of enlightened understanding.  In either case, it comes from the heart of man to His God and Lord.

There are two words used for praise which give a key to understanding the range of personal expression in the Psalms.  These two words translated praise are yadah and halal.  Yadah means “to confess,” literally “to stretch out the hand.”  The first use in the Bible is in Genesis 29:35.  Leah’s fourth child was named Judah, which means “praise the Lord.”  The first use of yadah in Psalms gives more insight into the depth of the word.

Psalm 7:17:
I will praise [yadah] the Lord according to his righteousness: and will sing praise to the name of the Lord most high.

The standard of what to confess or praise is according to God’s righteousness.

Whenever yadah is used, it is in reference to what God has done, either in mighty works or for an individual.  It is a description of what God has done.  In this sense it comes out of specific situations and is the confession of an individual’s heart of what God has done for them.  An example of this kind of praise is in Psalm 18:2ff.  The Lord is called a rock, a fortress or mountain stronghold, a deliverer, a high tower.  It is the confession of David of what God did for him and who God is to him.  God is described as “my strength, in whom I will trust.”

This confession or praise, yadah, becomes vividly understood because it is linked with thanksgiving.  It is translated 37x as thank or give thanks.

Psalm 18:49-50:
Therefore will I give thanks unto thee, O Lord, among the heathen, and sing praises unto thy name.  Great deliverance giveth he to his king; and sheweth mercy to his anointed, to David, and to his seed forevermore.

The second word for praise is halal and this word incorporates the idea of celebration and rejoicing.  We have the transliteration into English in hallelujah.  It is praise the Lord with an exclamation point!  This word when used often has to do with God’s mighty works of creation, his greatness, omnipotence.  There are five psalms called the Hallelujah Psalms, 145-150, which begin and end with “praise the Lord.”

These two kinds of praise are used together to describe the gamut of expression in the Psalms.  They are used together to describe the temple worship services.  Here are only several instances where the two words are used together:

1 Chronicles 16:4:
And he appointed certain of the Levites to minister before the ark of the Lord, and to record, and to thank [yadah] and praise [halal] the Lord God of Israel.
1 Chronicles 23:30:
And to stand every morning to thank [yadah] and praise [halal] the Lord, and likewise at even;
2 Chronicles 31:2:
And Hezekiah appointed the courses of the priests and the Levites after their courses, every man according to his service, the priests and Levites for burnt offerings and for peace offerings, to minister, and to give thanks, [yadah] and to praise [halal] in the gates of the tents of the Lord.
There is another word translated praise, but which is used together with singing, meaning to sing praises.  This word zamar and is translated to sing praises, to sing psalms.  It is not used separately from yadah or halal and covers both aspects praise.  Thus to sing praise is to put into music both confession and celebration.
Psalm 18:3, 49:
I will call upon the Lord, who is worthy to be praised: [halal] so shall I be saved from mine enemies.  Therefore will I give thanks [yadah] unto thee, O Lord, among the heathen, and sing praises [zamar] unto thy name,
The overall goal for writing of the Psalms is this personal expression of praise to our Lord and God.  Every name for God is used, to show all the different ways in which God works.
Psalm 18:28-32:
For thou wilt light my candle: the Lord [Jehovah] my God [Elohim] will enlighten my darkness.  For by thee I have run through a troop; and by my God [Elohim] have I leaped over a wall.
As for God, [El] his way is perfect: the word of the Lord [Jehovah] is tried: he is a buckler to all those that trust in him.
For who is God [Eloah] save the Lord? [Jehovah] or who is a rock save our God? [Elohim]
It is God [El] that girdeth me with strength, and maketh my way perfect.
The purpose of the Psalms is to glorify God and magnify his deliverance and love for His people.  We can praise (confess and thank) and rejoice (celebrate) for all God, our Jehovah Elohim, has done for us and have that temple worship in our day-by-day lives.
2 Chronicles 5:13:
It came even to pass, as the trumpeters and singers were as one, to make one sound to be heard in praising, [halal] and thanking [yadah] the Lord; and when they lifted up their voice with the trumpets and cymbals and instrument of musick, and praised [halal] the Lord, saying, For he is good; for his mercy endureth for ever: that then the house was filled with a cloud, even the house of the Lord.