There are three main types of relationships in Salesforce:
1. Master-detail relationships model parent-child relationships where child records cannot exist without a parent record and parent deletion deletes children. Up to two master-detail relationships are allowed per object.
2. Lookup relationships can link objects without implications for security, deletion, or required fields. Up to 25 lookups are allowed.
3. Many-to-many relationships are modeled using a junction object with two master-detail relationships to the original objects. This allows each record to link to multiple others in both directions.