[Avnish Yadav]

In simple words, API stand for Application programming interface.API is protocol thorugh which one application communicate with other. Salesforce can integrate with third party application and communicate data with API.

Salesforce API are :-

1. REST API
2. SOAP API
3. Tooling API
4. Chatter REST API
5. Apex REST API etc.

[Avnish Yadav]

For creating Many-to-Many relationship, we need Junction object. It is a custom object which can link one instance of a record to many children, and each child can equally be linked too. Junction Object have two master-detail relationships to two different record types