CRUD is access to Create, Read, Update and Delete records in an object. CRUD permissions are defined at the Salesforce user profile level and can be used to restrict the actions that users can perform on an object.

Field-Level Security (FLS) allows you (Administrator) to define the field-level permissions of objects for the users. You can provide Read or Write permissions to fields of objects. For example, you can set the Total Billable Amount field of the Work Order object in such a way that it is available to only those profiles that are part of the Service and Operations team.