Hallel of pesukei dezimra a selection of six psalms recited as part of pesukei dezimra (Hymnal Verses) — the introduction the daily morning service. This "Hallel" is specifically Ashrei (most of which is Psalm 145) followed by Psalms 146-150. The term Hallel without a qualifier general refers to Psalms 113-118 which are recited only of festivals; for this reason the Hallel of pesukei dezimra is also known as the "daily Hallel".
These psalms are recited because they are devoted entirely to the praise of God.
Originally, this was the only part of pesukei dezimra. Its recitation was considered voluntary for a long time.
Views
There are contradictory views on the daily recitation of Hallel. The Gemara refers to one who recites Hallel daily as a blasphemer. Talmud Bavli refers to one who recites it daily as praiseworthy. The recitation of these six psalms as an alternative to 113-118 is a compromise on this matter.