It depends on the malts in the recipe.
Generally, steeping is the process to extract the existing sugars from the malt and mashing is the process to convert starches in the malt into sugars. There are a number of categories of malts (base, crystal/caramel, roasted/dark, …).
@uberculture’s descriptions of the process are very common. Some kit makers write recipes where the crystal malts are steeped at 150* F for 20-30 minutes. I’ve done this type of steep a couple of times - and it did extract more sugars / flavors than a “put the bag in when starting to heat the water” steep. Whether or not “more is better” often depends on the goal of the recipe.
Some additional links / resources to consider:
- Expert Knowledge Base
- Cold Steeping: Getting the Most Out of Dark Grains - American Homebrewers Association
- The book Malt (Brewing Elements Series)
- The book Brewing Better Beer (Gordon Strong)
