Junction / Join Objects
A Junction/Join object is a special type of object that facilitates a many-to-many relationship by acting as an intermediary object containing the details of the relationship. These objects tend to be very light, simply containing two lookups, one pointing at each side of the relationship.
Below is an example use of a join object used within Seven20.
In general, join objects enable a huge amount of flexibility, but they tend to be outside of the end user's ability to manage. As such, they are best used in the background to facilitate the relationship between two objects, with custom components or automations driving the relationships instead of end users.
Join objects can utilise up to two master-detail relationships instead of lookups. However, when using master-details the same considerations need to be taken into account.
Unless absolutely sure, only use a single master-detail at maximum.