Odoo has 3 types of relations:
1) Many2one: A simple link to an other object.
    * Will be shown as a dropdown menu.
    * Can have a relation with tha same model to act as Parent/Child relation.
    * You can use related attribute to get access to a field in data in the model you are building a relation with.
2) One2many: A virtual relationship, inverse of a Many2one. A One2many behaves as a container of records.
    * Will be shown as a table with unique lines.
    * Needs a Many2one relation to work.
3) Many2many:  Bidirectional multiple relationship, any record on one side can be related to any number of records on the other side. Behaves as a container of records.
    * Will be shown as a table with multiple options that you can choose from and they are not unique to a model.
    * Have an optional reverse Many2many relation with the model you are tying to make relation with.
Read more about how Odoo relations work from Odoo Docs here.
Happy to help :) an upvote will be awesome