Similarities:
Similarity is that they are all Relational Database, and they all are proprietary software. Otherwise there exist no similarity. All of them and developed/owned by companies like IBM, Oracle and Microsoft and work totally different ways towards the same need, that is storing organized relational data and retrieving them easily.
Differences:
All have different user interfaces and use different scripting languages (Oracle - PL/SQL, SQL Server - T/SQL, Access - VBA, etc.). Most (if not all) also have a few add-ons to SQL (built-in functions and the like).
