These are the two database which got created when we run the SharePoint Products and Technologies Configuration Wizard (The databases are not automatically created when we install the product. The installation just install the binaries.) These are the master databases which stores all the config information for the farm.
· The configuration database (SharePoint_config by default)
· The Admin content database (SharePoint_AdminContent_GUID).
Every Web App has its own Content DB. All site collections which are there in same web app goes into this same content DB or individual site collection can be created into different content DB. See the post how to create Site Collection in different DB in the same web application here: Creating Site Collection in Different Content Database
Database created for an SSP: When we are creating any SSP within a new web app (which is a recommended practice) a new content DB is getting created for the Web App. If we host the ‘My Site’ feature into a new web app, again a new Content DB is created for My Sites. Two more DB’s are created for each SSP. One DB stores the data for the services provided by the SSP and another DB stores the search related data for quicker search performance.
You can also find interesting to read Some Important tables in SharePoint Content DB
Attempt a Question on SharePoint Databases