You have a Transaction table which is very important table because it contains all data of transacations that happend from beginning. You have a transaction log table.
If there is any modification done in transacations table automatically the old data for that row should be inserted in Transaction Log table. How will you achive this?

Trigger should be created. A trigger is a special kind of a store procedure that executes in response to certain action on the table like insertion, deletion or updation of data.
That too "After Trigger" should be created for "Transaction table". "After trigger" run after an insert, update or delete on a table.
As soon as modifcation made in Transaction table "After trigger" fired and should insert the row in Log table.

Instead Of Triggers used as interceptor for our table.If any one tries to modify our table which has instead of triggers, then instead of triggers will be fired.
Stored procedures are pre compiled statements for performing repetitive tasks
User-defined functions are routines that accept parameters, perform an action, such as a complex calculation, and return the result of that action as a value and table format.