From: Wikipedia

The text is organized in 10 books, known as Mandalas, of varying age and length.

• The "family books":
mandalas 2–7, are the oldest part of the Rigveda and the shortest books; they are arranged by length and account for 38% of the text.
• The eighth and ninth mandalas,
comprising hymns of mixed age, account for 15% and 9%, respectively.
• The first and the tenth mandalas
are the youngest; they are also the longest books, of 191 suktas each, accounting for 37% of the text.

Each mandala consists of

• hymns called sūkta
(su-ukta, literally, "well recited, eulogy") intended for various sacrificial rituals. The sūktas in turn consist of
• individual stanzas called ṛc
("praise", pl. ṛcas), which are further analysed into
• units of verse called pada
("foot").
• The meters most used in the ṛcas are the
• • jagati (a pada consists of 12 syllables),
  • trishtubh (11),
  • viraj (10),
  • gayatri and
  • anushtubh (8).

For pedagogical convenience, each mandala is synthetically divided into roughly equal sections of several sūktas, called anuvāka ("recitation"), which modern publishers often omit. Another scheme divides the entire text over the 10 mandalas into aṣṭaka ("eighth"), adhyāya ("chapter") and varga ("class"). Some publishers give both classifications in a single edition.

The most common numbering scheme is by book, hymn and stanza (and pada a, b, c ..., if required). E.g.,

• the first pada is
1.1.1a agním īḷe puróhitaṃ "Agni I invoke, the housepriest"
• and the final pada is
10.191.4d yáthā vaḥ súsahā́sati