what's the use of the alpha value of a texture

we know when a texture has 4 color components,the last one is alpha,but what’s the use of it?which objects will be affected by it?OGL how to deal it in a blend operation?

please ask such questions in the beginner forum.

I would like to know the answer to this question as well. Is there a performance difference between GL_QUAD and GL_TRIANGLES

I somehow managed to post that in the wrong thread… sorry. I have no idea how that happened.