It's pretty complicated.
The amount of misinformation in this thread is staggering. You guys seem to have no idea what you are talking about. Stop confusing the OP. I'll explain it.
Below is a very technical explanation:
The XBox 360 has DirectX 10, as you all know, but it actually uses a special version that utilizes some features planned for DirectX 11 (known internally to Microsoft as DirectXXI ). That software has a special hardware function called Ebkac that enables real-time NURBS and raytracing through advanced box-modelling techniques. That's why 360 games look a little better; the software lets the 360 pixel-draw with much less CPU cost. Plus, the 360 has three 3 GHz G5 processors, it's like a PowerMac.
Meanwhile, the PS3 has its own advantages. See, the Cell processor can put out teraflops, it has 8 processors, and uses its teraflops to texture multi-sided polygons, it's really quite impressive. That's where the term 4D comes from. The cell's speed is rated at 120,000 HPC. The 7th Synergistic Processing Element (#007) in the Cell processor runs a process called RSX that is used for graphical effects.
Both have problems though. The 360 uses Shared RAM, and anyone who has ever built a PC knows that shared RAM is slower than dedicated. But the PS3 doesn't have a graphics card, using the Cell for everything, and developers don't know how to use the Cell very well, so 360 games always end up looking better.
I hope you learn from this, it's really quite fascinating the way this industry works.
Some people might spin stories about one system being better or worse than the other, just ignore them. Pay close attention to this post, I explained everything, don't listen to anyone else.