Winter is a great time to get your weed problem under control. Once your grass goes dormant, the lawn weeds stick out, as they are the only green left. White clover and sow thistle are common weed problems found in the winter and are fairly easy to treat by readily available broad spectrum selective herbicides.
Poa annua, annual bluegrass, is a grassy annual winter weed. This weed dies off each year, but leaves hundreds of seeds per weed to grow back the following year. The solution for this weed is to use a good pre-emergent before it starts growing. Most treatments after this weed has started growing are effective at warmer temperatures when it would be close to dying off on its own anyway.