The definiton of a transaction in a database is an operation that can easily reversed.
Lets imagine that you have a huge database of items that are open for sale, and suddenly you need to dramatically change the price list. Good example would be transfer to Euro.
So you have a few thousand records, and you have a formula to translate prices from one currency to the other. You begin transaction, modify all those records, and only after you are 100% sure that all the changes are correct, you commit the changes made in the transaction to the database. If there was an error, you can roll back all the changes, and restore the database to the previous state.
How different database servers handle transactions, where do they store the changes, and how they perform the operation is a different story, but this explanation should give some insight to transactions in general.
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