what is difference between 3D and 2D graphics??

how we can identify the 3D and 2D graphics???

Assuming you don't mean "3D" in the sense that the picture jumps out of the screen while wearing 3D glasses, such as in many newer films being released to theaters, then the difference is in how the images are generated. Older Disney films like Snow White are 2D: flat drawings. Newer Disney films (working with Pixar) like Toy Story are 3D. To the audience they are all 2D (unless presented with 3D glasses, which until recently was rare), but the graphics look different. The main difference is that there can be rotation of objects in 3D graphics and not in 2D. But a lot of people still like the look of 2D "cartoons" more.

Realistic graphics are usually 3D, such as adding "computer generated [imagery]" (CGI) characters to live action films.

But for web design you rarely will need to use 3D methods. 2D graphics are usually enough.

In Photoshop and Illustrator, in addition to X (horizontal) and Y (vertical) axis, there is simulated Z axis for depth and -- of course -- the appropriate tools to rotate and transform along the 3 axes.

the basic difference between 3D and 2d is that 3D gives us real like image. The image is so real that it creates an illusion of reality. We feel that we can touch it. On the other hand in 2d picture is not so real and we feel it as imaginary.