PART 1 -
The Categories of Hajj

1. The Hajj is either obligatory or mostahab (desirable, optional), and the obligatory Hajj is classified in three categories:

        I.  Hajjat-al-Islam, which is obligatory upon the individual when s/he meets all the prescribed criteria, which shall be mentioned shortly InSha’Allah.  According to the fundamentals of Islam, the Hajj is not obligatory except once.

        II.  The Hajj that becomes obligatory through nadhr (vow), promise, and oath.

        III.  The Hajj that becomes obligatory by proxy, thorough the hiring of agents.
As for the mostahab Hajj, it is any Hajj other than the above.
The Conditions of Obligation of Hajjat-al-Islam

2. The conditions that if met make Hajjat-al-Islam obligatory are five:
   a.  Adolescence,
   b.  Sanity of the mind,
   c.  Freedom,
   d.  Ability – to be able to afford going to the Hajj and come back, as well as leaving behind enough for his family to live on, “Returning to Sufficient” means of living.
   e.  Absence of hindrance on the way [to the Hajj].
3. The obligation of the Hajj is immediate when its requirements are all met. That is, if the individual meets all the preconditions and prerequisites of the Hajj – thus considered “able” and “eligible”, or in technical terms mostatee’ [8] – it becomes compulsory upon him [to go to Hajj] in the same year of eligibility, and it is not permissible for him to delay [going to Hajj].  Delaying the Hajj without having a valid [Islamically] legal reason constitutes a sin, and [from then onwards] the Hajj remains “established” as his responsibility [regardless of his ability or the lack of it].  It is mandatory for him to perform the Hajj in the following year [and if not] as soon as possible.
4. It is obligatory upon the mostatee’ to prepare all the necessary means and requirements of the Hajj journey in the first year of his ability/eligibility, even if they were many and lengthy.
5. It is obligatory upon the individual going to Hajj to learn the Hajj rites and rulings.
6. If it is not possible for him to go to Hajj on his own, he may choose a group that he trusts to go with, and if there are many groups that he trusts, it would not be obligatory for him to go with the first one.  If he travelled with other than the first one, but, due to certain circumstances, he did not manage to get to the Hajj; if in the future, he were not to meet all the prerequisites of the Hajj, he would not be regarded as having committed disobedience, nor the Hajj remains “established” in his responsibility.
7. If he travelled with a group that he is not sure of, and it happened that he did not get to Hajj, he has sinned, and the Hajj remains “established” in his responsibility.  It is obligatory for him to perform the Hajj in the following year, or as soon as possible.