System Database

master

Records all of the system level information for a SQL Server system.
It records all login accounts.
Records the existence of all other databases, including the location of the database files.
Records the initialization information for SQL Server;
Always have a recent backup of master available.

tempdb
tempdb holds all temporary tables and temporary stored procedures.

tempdb is a global resource; the temporary tables and stored procedures for all users connected to the system are stored there.
tempdb is re-created every time SQL Server is started so the system starts with a clean copy of the database. Because temporary tables and stored procedures are dropped automatically on disconnect, and no connections are active when the system is shut down, there is never anything in tempdb to be saved from one session of SQL Server to another.
By default, tempdb autogrows while SQL Server is running.
Unlike other databases, however, it is reset to its initial size each time the database engine is started.
If the size defined for tempdb is small, part of your system processing load may be taken up with autogrowing tempdb to the size needed to support your workload each time to restart SQL Server.
You can avoid this overhead by using ALTER DATABASE to increase the size of tempdb.
model
The model database is used as the template for all databases created on a system.
When a CREATE DATABASE statement is issued, the first part of the database is created by copying in the contents of the model database, then the remainder of the new database is filled with empty pages. Because tempdb is created every time SQL Server is started, the model database must always exist on a SQL Server system.
msdb
This is used by SQL Server Agent for scheduling alerts and jobs, and recording operators