A Sookt is a hymn addressed to a Vaidik Deity. A Vaidik hymn consisting of many verses which are called Mantra on various occasions like sacrifice, worship and so on. Rig Ved is constituted of 10 Mandal, and the Mandal are constituted of several Sookt. In the Ved, elements of nature, like stone, tree, morning etc were also called Deities.

There are five main Sookt -
(1) Bhoomi or Prithvi Sookt to please Prithvi,
(2) Purush Sookt to please Naaraayan,
(3) Rudra Sookt to please Rudra - Rudra Sookt seems to be only one Shlok, Atharv ved, Ch 7, Sh 92 ( Ramana Maharshi site)
(4) Shree Sookt to please Lakshmee, and
(5) Rig Ved Sookt - according to Bhavishya Puraan, 3/22 - "Praanshu satisfied Soorya Dev with Rig Ved Sookt".

But there are some other Sookt also - 

Balittha Sookt of the Rig Ved --
Aayushya Sookt
Devee Sookt, (Rig Ved, X.125.3-4)
Durgaa Sookt
Medhaa Sookt,
Mrityu Sookt
Naaraayan Sookt,
Naasadeeya Sookt,
Nakshatra Sookt,
Pavamaan Sookt
Samvaad Sookt - It is in the form of dialog between Yam and Yamee. It is found in Rig Ved and is generally considered as a Sookt to bring harmony between couples.
Shiv Sookt,  
Vaayu Sookt,
Vishnu Sookt,
Yaamya Sookt - Raam recited it at the time of Jataayu's death.
Yam Sookt,

Bhaagya Sookt - in Horaa Ratn Maalaa