There are two main types of thermodynamic processes: reversible process and the irreversible process.
The process is said to be an irreversible process if it cannot return the system and the surroundings to their original conditions when the process is reversed. The irreversible process is not at equilibrium throughout the process.
For example, when we are driving the car uphill, it consumes a lot of fuel and this fuel is not returned when we are driving down hill. Many factors contribute in making any process irreversible. The most common of these are
2) Unrestrained expansion of a fluid
3) Heat transfer through a finite temperature difference
4) Mixing of two different substances.
The basic concept is that most of the thermodynamic processes have a preferred direction just as Heat always flows from hotter object to colder object. Once a gas is released in a room, it expands in room and never contracts without indulgence of any external force etc.
But in some systems, the reverse occurs. Normally it happens when that system is close to thermal equilibrium. This equilibrium has to be inside the system itself and also within the system and its surroundings. When this stage is reached, even a small change can change the direction of the process and therefore such a reversible process is also known as an equilibrium process.
A very simple example can be of two metal jars A and B which are at a thermal equilibrium and are in contact with each other. Now when we heat jar A slightly, heat starts to flow from Jar A to Jar B. This is the direction of this process. Now this process can be reversed just by cooling Jar A slightly. When Jar A is cooled, heat flows from Jar B to Jar A till thermal equilibrium is reached.