What's the difference between a PM and steeping grains?

What are the differences between these processes? I have done both but they really seem about the same to me.

What does a higher or lower Mash temp do? More sugars?.?.? or less?.?.?. I have been staying around 150-155 for my PM.

Is mashing more effective than steeping?

Specialty grains that are steeped have already had the starches converted to sugars during the roasting process, to put it in its simplest terms. During the steep all you are doing is removing these sugars, colors and flavors. During a mash of base malts, the heat activated enzymes in the malt convert the starches to fermentable sugars. Low mash temps make a more fermentable wort, leading to a more dry beer, while higher mash temps will give a more malty character. These explanations are simplistic, but give you a good idea of what is happening.

Steeping grains adds some flavor and color.

Partial Mashes add fermentable content along with the color and flavors.

edit: I should add that is a general guideline....