What is Abstraction?

Abstraction is one of the 3 pillars of Object Oriented Programming (OOP). It literally means to perceive an entity in a system or context from a particular perspective. We take out unnecessary details and only focus on aspects that are necessary to that context or system under consideration.

Here is some good explanation:

You as a person have different relationships in different roles. When you are at school, then you are a "Student". When you are at work, you are an "Employee". When you are at government institution, you can be viewed as a "Citizen". So it boils down to what in what context are we looking at an entity/object. So if I am modelling a Payroll System, I will look at you as an Employee(PRN, Full Time/Part Time, Designation). If am modelling a Course Enrollment System, then I will consider your aspects and characteristics as a Student(Roll Number, Age, Gender, Course Enrolled). And if I am modelling a Social Security Information System then I will look at your details as a Citizen(like DOB, Gender, Country Of Birth, etc.)

Difference between Encapsulation and Abstraction:

Remember that Abstraction(focusing on necessary details) is different from Encapsulation(hiding details from the outer world). Encapsulation means hiding the details of the object and providing a decent interface for the entities in outer world to interact with that object or entity. For example, if someone want to know my name then he cannot directly access my brain cells to get to know what is my name. Instead that person will either ask my name. If a driver wants to speed up a vehicle then there is an interface(accelerator pedal, gear, etc) for that purpose.