At it's core, Salesforce is basically a database. You can create custom objects (tables) and fields (columns) tailored to your business' needs to store anything and everything. But you can't just easily replace it with a database because they have tons of layers of automations and workflows built on top to make it insanely user friendly: Customer sends an email and it's automatically logged and tickets opened, sales person has a call and can create quotes and they are automatically sent to the correct people for approvals, managers can get access to accounts managed by their team but not the entire company, etc. It's the "works out of the box but still let's you customize them" business process automations and UI that make Salesforce what it is.