As alluded to above... the temperatures refer to the concept of mashing versus steeping grains. For an extract brewer to accomplish a more complete taste they will steep grains in hot water.. this water should not exceed 170F or you run the risk of extracting tannins. Why 150F water could be appropriate for some invovles the concept of mashing.. read on if you like.
Mashing basically refers to the process whereby enzymes present in certain grains will convert the starches in the barley, wheat, oats, pumpkin, potato, etc. to (mostly) fermentable sugars. Through this process you end up with a sugar rich fermentable liquid that the yeast transform into beer. There are various enzymes that do various things and they are each active at various temperatures and PH ranges. It will suffice to say that for the all grain brewer a ph in the range of 5.2-5.3 and a temp (in an infusion mash) of 153 is a generally accepted compromise. If you were doing a true mini mash as some around here do then you'd let the grains soak in water at this temp for 50 or 60 minutes - this process would result in a more "beer like" complex flavor as well as some fermentables because the enzymes effectively converted the starches to sugar. Now, if all you're doing is soaking the grains in hot water then hotter water will probably do a better job of rinsing sugar from the grains. This is, of course, assuming that you're using grains suitable for this purpose; suitable grains are those that have already had a lot of their starches converted to sugar through the malting process. Any enzymatic activity that would occur (assuming you're using grains that are enzyme rich) will be quickly halted at these higher temps as the heat will quickly denature them... this is no concern though because all you're expecting is some flavor and color and few, if any, fermentables. That's the long end of it.
Here's the take-away: If you're using grains that have already had a lot of starches converted (i.e. crystal malt) then steeping them in 170F water will remove these sugars but will not convert any remaining starches to sugar b/c the enzymes were denatured by the heat. You won't get a lot of fermentables from this process but will get a good amount of flavor. Hotter water (to a point) is better. If, on the other hand, you're using grains that have not been malted and are very starchy (i.e. oats or wheat) then simply soaking them in hot water will do nothing because there are no residual simple sugars present.. only starches. For these grains they will have to be mashed (150F) in the company of some base malt (2 row or 6 row) that is enzyme rich. The base malt will provide the enzymes needed to convert the starches in the oats or wheat to sugar so that you can then extract it and the yeast can ferment it. So, to answer your question, the appropriate temperature is part of a larger process - steeping vs. mashing - and which process you should use is really dependant on what type of grains/adjuncts your using and what you want to get out of them (and, of course, what equipment you have handy).
Hope this helps..
