Samhita [collection] of each ved is organized in-terms of Mandals. Mandals are organized as Anuvakas, which are organized as Sukts which contain Mantrs [riks]. The organization is shown here. Anuvakas are not commonly used.

Rig-Ved Samhita has Ten Mandals each with different number of sukts and mantrs. Please visit following sites for more knowledge.

URL Comment
Vedicscriptures - Rig-Ved Language - Hindi and English; navigate thru the site learn about rig-ved Mandals/ Sukts/ Matrs as well as many translations and commentaries. You can also download some of the translation and commentaries. Navigating the content is very user friendly.
Vedicheritage - Rig-Ved: Samhitas

A GOI site. Includes links to available Rig-Vedic Brahmans, Arayaks and Upanishads. You can also listen to the riks. Overall, excellent source of information.

Hindupedia - Rig-Ved

Another excellent site, look at the table of contents slightly below the top of the page.

 Google and YouTube will list many other sources, choose wisely, some are biased and have an agenda.