Can someone explain the difference between using steeping grains and partial mash?
Mashing is temperature-dependent. Steeping is not.
Mashing is the activation of enzymes in malt which has diastatic power - the power to convert starches to sugars. These enzymes are activated and deactivated in temperature ranges. Too cool, they'll never activate. Too hot, and they'll be denatured (killed). The ratio of water to grain (liquor to grist) is important also. Basically, the intent of mashing is to convert starches to fermentable sugars.
Steeping is simply leaching flavor, aroma and color compounds into the brewing liquor (brewers only use
water to clean and rinse; if water goes into the beer, it's called
liquor!). It's exactly like tea - you dunk a teabag into hot water, and the color, flavor and other stuff are leached into the water. When steeping grains, you use the same principle, just with different stuff in a larger bag.
Because you don't necessarily concern yourself with controlling the temperature, it isn't the same as mashing.
Now I shall briefly complicate things. Crystal malt has already had its starches converted to sugars in the malting process. That process is why it's called crystal (or caramel) malt. Simply steeping crystal malt will leach some of its sugar into the liquor through dissolving. Not as much as mashing, but some.
Cheers,
Bob