The User Profile is mandatory when creating a user. It defines what the user can view and do within the environment. It contains among others App Permissions, Object Settings and Permissions, System Permissions, Custom Permission, Assigned Connected Apps, Salesforce Access Conditions (IP restriction, login hours restriction).
With standard CRM, there are 6 main standard profiles: System Administrator, Standard User, Solution Manager, Marketing User, Contract Manager, and Read Only. There are also 3 standard profiles Chatter: Chatter Free User, Chatter External User, Chatter Moderator User.
At times you may want to clone profiles to create similar ones and customize them as needed.
A user can only have one profile.
If a user needs additional permissions, you do not need to change his profile (he may not be the only user with this profile), you must create and assign Permission Sets (that are handled per user).
A profile is not created from scratch, it is necessarily cloned from a standard profile.