Transactions in Databases

Transaction is databases is executing set of database instructions in a sequence which should be accomplished together. Transactions have following properties which are known as ACID (wiki).

• Atomicity : Ensure all operations are completed successfully or in a failure, all operations are rolled back to its previous state
• Consistency : Ensure database properly changes state upon a successful transaction completion
• Isolation : Ensure transactions are operate independently 
• Durability : Ensure result or effect of a committed  transaction persist in a case of a system failure

Transactions in Entity Framework

Whenever you execute SaveChanges(), the framework will wrap that operation in a transaction. Starting with EF6, Database.ExecuteSqlCommand() by default wrap the command in a transaction if one was not already present. Entity framework does not wrap queries in a transaction.

Read below articles for more information Working with Transactions (Data Developer Center)
Managing connections in Entity Framework (Visual Studio)
Using Transactions or SaveChanges (SO)

Transactions in SQL Server

Implicit Transaction 
http://dba.stackexchange.com/questions/43254/is-it-a-bad-practice-to-always-create-a-transaction
http://www.codeproject.com/Articles/4451/SQL-Server-Transactions-and-Error-Handling
http://stackoverflow.com/questions/10153648/correct-use-of-transactions-in-sql-server-2008
http://www.tutorialspoint.com/sql/sql-transactions.htm
http://www.dotnet-tricks.com/Tutorial/sqlserver/c2XF120412-SQL-Server-Transactions-Management.html


SAP
https://help.sap.com/saphelp_gateway20sp08/helpdata/en/41/7af4bca79e11d1950f0000e82de14a/frameset.htm