The name's self explanatory, it's any pack big enough to carry everything you need for a day in the hills. Variations centre on volume, materials, types of back system, shape and general fit. Your choice will depend on the use you intend to put the pack to - adventure racers will want a small lightweight pack; climbers will want a slim strealined one; and there are women's specific packs out there too.
Daysacs - Buyer's Guide
General hill-walking sac essentials, from pole holders to waterproofing options.
Hands on shop floor advice if you're buying a day sac - a bad one can ruin your day, so pay attention now... 
