SQL Server provides a number of global variables, which are very useful.The values of these variables is maintained by the server. All the global variables represent information specific to the server or a current user session.The names of global variables begin with @@ prefix.Following are the major global variables :

1) @@CONNECTIONS

It returns the number of logins/(attempted logins) since SQL Server was last started.

Return type: int

2) @@MAX_CONNECTIONS

It returns the the maximum number of simultaneous connections that can be made with SQL Server in this computer environment. The user can use (sp_configure ''number of user connections'') to configure SQL Server for any number of connections less than or equal to the value of @@max_connections.

Return type: int

3) @@CPU_BUSY

It returns the amount of time(in ticks) that the CPU has spent doing work since the last time SQL Server was started.

Return type: int

4) @@ERROR

It is commonly used to check the error status of the most recently executed statement. It contains 0 if the previous transaction succeeded, else it contains the last error number generated by the system.

Return type: int

5) @@IDENTITY

It returns the last value inserted into an IDENTITY column by an INSERT or SELECT INTO statement. @@identity is reset each time a row is inserted into a table. If a statement inserts multiple rows, @@identity reflects the IDENTITY value for the last row inserted. In case the affected table does not contain an IDENTITY column, @@identity is set to 0.

The value of @@identity is not affected by the failure of an insert or select into statement, or the rollback of the transaction that contained it. @@identity retains the last value inserted into an IDENTITY column, even if the statement that inserted it fails to commit.

Return type: numeric(38,0)

6) @@IDLE

It returns the amount of time(in ticks) that SQL Server has been idle since it was last started.

Return type: int

7) @@IO_BUSY

It returns the amount of time(in ticks) that SQL Server has spent doing input/output operations after it was last started.

Return type: int

8) @@LANGID

It returns the local language id of the language currently in use , specified in syslanguages.langid.

Return type: smallint

9) @@LANGUAGE

It returns the name of the language currently in use ,specified in syslanguages.name.

Return type: nvarchar

10) @@MAXCHARLEN

It returns the maximum length, in bytes, of a character in SQL Server's default character set.

Return type: tinyint

11) @@PACK_RECEIVED

It returns the number of input packets read by SQL Server from the time it was last started.

Return type: int

12) @@PACK_SENT

It returns the number of output packets written by SQL Server from the time it was last started.

Return type: int

13) @@PACKET_ERRORS

It returns the number of errors that have occurred during sending/receiving of packets by SQL Server.

Return type: int

14) @@ROWCOUNT

It returns the number of rows affected by the last command.In case the command(e.g IF) does not return any rows, @@rowcount is set to 0. For cursors, @@rowcount represents the cumulative number of rows returned from the cursor result set to the client, up to the last fetch request.

Return type: int

15) @@SERVERNAME

It returns the name of the local SQL Server.The server name must be defined with sp_addserver following which SQL Server must be started.

Return type: varchar

16) @@SPID

It returns the server process ID of the current process.

Return type: smallint

17) @@TEXTSIZE

It returns the current value of the set textsize option, which specifies the maximum length, in bytes, of text or image data to be returned with a select statement. Defaults to 32K.

Return type: smallint

18) @@TIMETICKS

It returns the number of microseconds per tick. The amount of time per tick is machine dependent.

Return type: int

19) @@TOTAL_ERRORS

It returns the number of errors that have occurred during read/write by SQL Server.

Return type: int

20) @@TOTAL_READ / @@TOTAL_WRITE

It returns the number of disk reads by SQL Server from the time it was last started.

Return type: int

21) @@TRANCOUNT

It returns the nesting level of transactions. Inside a batch each begin transaction increments the transaction count. In case you query @@trancount in a chained transaction mode, its value will never be zero becaue the query automatically initiates a transaction.

Return type: int

22) @@VERSION

It returns the current version of SQL Server.

Return type: nvarchar

Ref : https://code.msdn.microsoft.com/Global-Variables-in-SQL-749688ef