There are three types of ihram and they are as follows:

Tamattu`: At or before reaching the miqat (the place where one must enter ihram), the person enters into ihram for `Umrah only. He performs `Umrah first, then goes out of ihram until 8 DhulHijjah, when he or she enters into ihram again for Hajj. This is the most common type for people from outside of Saudi Arabia.
Qiran: The person enters into ihram for both `Umrah and Hajj, and does not go out of ihram until the Day of Sacrifice (`Eid Al-Adha) at Mina. In this type of Hajj, the pilgrim has to stick to the restrictions of ihram for a longer period of time.

Ifrad: The person enters into ihram only for the Hajj and takes it off only on the Day of Sacrifice.

Sheikh SayyedSabiq, states in his well-known book, FiqhAs-Sunnah: If a person puts on ihram with the intention of performing whatever Allah has prescribed for him without specifying any of the three kinds due to lack of knowledge, his ihram is considered to be lawful and valid.

Scholars have said: “If such a person utters Talbiyah like others with the intention of performing the rituals (of Hajj), but says nothing verbally, nor forms an intention in his heart, nor specifies whether it is tamattu‘, ifrad, or qiran Hajj that he intends, his pilgrimage would still be valid, and he will be reckoned to have made one of the three kinds of Hajj.