we have classes even among vedic rituals !

That which fulfills your desire is called Ishti. In a broader sense ishti is a yajna but when the yajna is done for just a few hours in a single day to fulfill one's minor desires, it is called ishti. Ishti is done to please the God that can fulfill those particular desires.

The different yajna's depending on the actual time you spend on them are classified as:

1. Ishti (1 or 2 hours, e.g., Lakshmi-Narayana Ishti, Sudarshana Ishti, Narasimha Ishti, Hayagriva Ishti (for better studies), Vishwaksena Ishti (to remove obstacles) and Vaibhava Ishti (for wealth)
2. Homam (1 day)
3. Yajna (3, 5, 11 days or 3 months)
4. Maha-yaagam (1 or 2 months)
5. Advaram (6 months or 1 year)
6. Satram (50, 100, 500 years or 1000 years)