Various holds may show up on your account preventing you from registering for classes.  Please note that even if you have a hold you can still drop classes if it's before the withdrawal deadline.
Below are the most common:

HoldContact
Accounts Receivable 306-966-4595 or student_accounts@usask.ca
Agency Hold 306-966-4595 or student_accounts@usask.ca
Admission 306-966-5788 or admissions@usask.ca
Graduate Studies 306-966-5751 or grad.studies@usask.ca
Library 306-966-5958
Residences 306-966-6775 or residence@usask.ca
Student Health 306-966-5768
Academic Standing Contact your College
Student Status 306-966-1212

If you have questions about these holds or any other holds contact Student Central.