Firstly: The Definition of Release
Linguistically, tahallul (release) is when the restrictions which are haram for someone during Hajj become halal. A man is called haram if he is not in a state of ihram or engaged in the means of Hajj. A similar verb is used to refer to someone who leaves the Sacred Precinct (al-Haram).
Technically, tahallul is release from ihram along with the permissibility of engaging in what was haram for him during ihram.
Secondly: What Causes the First Release
When one pelts Jamrat al-`Aqabah and shaves or cuts their hair, they undergo the first release.  Some scholars hold that release only occurs by pelting and shaving to the exclusion of everything else. This is the position of the Hanafi school. This is the chosen position of al-Shinqiti and Ibn `Uthaymin. Shafi`is and Hanbalis hold that the first release is effected by performing two out of three actions: pelting, shaving, and circumambulation. This is the chosen position of Ibn Baz. Some hold that it is effected by pelting Jamrat al-`Aqabah. This is the position of the Maliki school, one position amongst Shafi`is, and a narration from Ahmad. This is the chosen position of Ibn Qudamah and al-Albani. Consensus on this was related by Ibn `Abd al-Barr, al-Nawawi, and Ibn Taymiyyah.
Thirdly: Consequences of the First Release
The first release permits everything that was haram for one except for women. This is the position of the majority: the Hanafis, Shafi`is, and Hanbalis. It is also the position of a group of the Salaf.
Fourthly: What Causes the Second Release and What its Consequences Are
If a pilgrim performs the Mass Circumambulation after completing the actions of Hajj, they have thereby undergone the second (greater) release. Everything is now permissible for him, including women. Consensus on this was related by Ibn Hazm, Ibn Hajar al-Haytami, and al-Shirbini.