Freeze:
If an user is freezed,

1. He will not be able to login.
2. User license of that user will not be released.

This is mainly used if you want to restrict an user to login during any activity.

Check the below link for Freezing and Unfreezing.

https://www.infallibletechie.com/2013/10/freeze-button-in-salesforce-user-detail.html

Inactivate:
If an user is inactivated,

1. He will not be able to login.
2. Report will not run, if he/she is set as running user.
3. Web to lead and web to case will not work, if he is set as the default creator.
4. User license of the inactivates user will be free for other user.

This is mainly done, when the user leaves the organization.

Cheers!!!