Step 1: Name the marriage forms:
The eight recognised types are Brahma, Daiva, Arsha, Prajapatya, Gandharva, Asura, Rakshasa and Paishacha vivah, varying in ritual and consent.
Step 2: Explain exogamy with its purpose:
Exogamy means marrying outside one's own clan/gotra; it was practised to avoid consanguineous unions and to build alliances between different kin groups.
Final Answer:
\[ \boxed{\text{Marriage outside one's own gotra/clan is exogamy}} \]