Texture2D is a bit of example code from Apple for loading textures from files and drawing them to the screen using OpenGL.
If you're creating a game, you have two main choices - UIKit (simple to learn, generally slow and memory-intensive, 2D only) and OpenGL (difficult to learn, based on C, but draws fast and can draw in 3D.)
I usually suggest UIKit for making your first game, and using a 2D library like Cocos2D (a wrapper for OpenGL that lets you draw 2D sprites easily) after that. Learning OpenGL is great if you're going to write engines for a living, but you'll spend a lot of time on "plumbing."
thx you, now i m understand but what do you mean by plumbing ???
I mean if you decide to write your own OpenGL code then you'll spend a lot of time writing code to render sprites, render text, load and cache textures, draw animations, and other behind-the-scenes work that is not related to gameplay. Those features are already built into Cocos2D and UIKit, so if you use those libraries you'll spend more time on gameplay programming and less time on "engine work."
OpenGL is difficult to learn and is based on C, but draws fast and can draw in 3D. Cocos2D is a wrapper for OpenGL that is, in my opinion, easier to learn. It already contains much of the "plumbing" code that I was talking about.
texture2D for game
thx for the patience you really a great help for amateur like me 
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