this is the rotten egg smell that you get in cider frequently (and some lagers during primary ferment)

many think this ages out, and that can be true, but one month after you pitched your yeast, if you still smell it after racking, you have a problem that won't go away on its own.

several things work to remove it.
essentially, you need to release it as a gas. this means slightly oxidizing your cider or cyser or anything with apple juice in it. as you oxidize, you lose body. this is no good.
copper will remove it, as it binds it up and then settles out. problem is, copper is toxic and you need to use small amounts, plus it also removes body.

here is what i do:
stir with a small diameter, copper tube i picked up from the hardware store for 5 bucks. i stir vigorously for 3-5 minutes, getting some air into it and disturbing the lees on the bottom to make it all cloudy. the slight oxidation will remove body, but stirring up the fine lees will add more body back in.
after doing this, check again in one week. you almost certainly won't smell it. wait.
check again 3 weeks after stirring, often it comes back and you need to repeat this again. if its all good after three weeks, you should be set.

to avoid its creation in the first place, keep the yeasties happy, make sure the campden you use is the PPM level you are figuring. i recently added campden thinking it was 50 ppm when it was actually 150 ppm! this causes a very clean must, but will usually cause these problems later on as the yeast is incredibly stressed when it finally begins fermenting.