Hello CGPA means Your total percentage divided by 10.
For example Your CGPA is 8 then You have secured 80% marks.
Each subject have some credits which is multiplied by pointer you scored In that subject and sum of all subjects multiplied by their respective pointer a d then divided by total credit represent  CGPA.
For example
Maths has 2 credits and Physics have 2 credits
And I scored 8 pointer ( generally 70-79 Mark's means 8 pointer) in maths and 7 pointer in Physics then my cgpa will be
======(2*8 +   2*7)/4
I hope it helps.

Your grades are allotted to you by dividing the 100 marks into 7 slabs. These slabs are named A+, A, B, C, D, E and for those who have failed it is F.
A+ is for an extra ordinary performance
A for a good score
B for an above average score
C for an under average score
D for a bad score
E for a close shave
F for failed.

Well so these grades have certain numeric values... 
A+ = 10
A = 9
B = 8
C = 7
D = 6
E = 5

Hi aspirant

Like we have marking system in 10 i.e CGPA in 12th Percentage in the same way in B.Tech CGPA is a kind of marking system.

If marks lies in between  90-100  ---A+  Grade Points:10

                                     80-90  ---A     Grade Points:9

                                     70-80  ---B+  Grade Points:8

                                     60-70  ---B    Grade Points:7

There is formula to calculate CGPA : Sumession of(Ci Pi)/sumession of(Ci)

Ci is the number of credits you have earned in ith course.

Pi is the grade points you have earned in the ith course.

Hope this was helpful.

hello aspirant,
CGPA stands for cumilative grade point average in engineering ,not only in engineering field, CGPA'S are caluculated in different types of education courses.
well, it is calculated on the basis of overall academic performance of students.
thank you