A transaction can be defined as a group of tasks. A single task is the minimum processing unit which cannot be divided further.

Let’s take the example of a small and simple transaction. Suppose a bank employee transfers Rs 1000 from P's account to Q's account. This very small and simple transaction involves several low-level tasks.

P’s Account

Open_Account(P) 
Old_Balance = P.balance
New_Balance = Old_Balance - 1000
P.balance = New_Balance
Close_Account(P)

Q’s Account

Open_Account(Q)
Old_Balance = Q.balance
New_Balance = Old_Balance + 1000
Q.balance = New_Balance
Close_Account(Q)

ACID Properties
A transaction is a very small unit of a program and it may contain several low-level tasks. A transaction in a database system must maintain Atomicity, Consistency, Isolation, and Durability − commonly known as ACID properties − in order to ensure accuracy, completeness, and data integrity.