What are immigrant visas?

Immigrant visas are those visas which give you a permanent residence instead of just an authorization to work. They can be based on family petitions, employment petitions, investment or humanitarian petitions. Examples are the

Family Immigration
1. Marriage Petitions
2. Parents Petitions
3. Sibling Petitions

Employment Immigrant visas
1. EB1 Visas – EB1A for individuals of extraordinary capabilities, EB1B for outstanding researchers, scientist and researchers, EB1c for multinational executives and managers
2. EB2 Visas – Through the process of labor certifications or through National Interest Waivers.
3. EB3 Visas – Through the process of labor certifications
4. EB4 Visas – Religious visas
5. EB5 – Investor of $500000 or $10000000 resulting in first a conditional permanent residence then a full permanent residence.

Humanitarian Immigrant Visas
1. Green card through Asylum
2. Green card through VAWA
3. Green card through U Visas

The above are just few options, they are just the common ways to get a greencard (immigrant visa) to the United States.