A squall line looks pretty much the same as a thunderstorm rolling in, the clouds are dark, moving quickly and the temperature will sudden drop creating a noticeable difference and the wind will pick up speed quickly.
A squall line is an unexpected increase in the speed of the wind by 20 miles per hour with gusts up to 25 miles an hour and sustain at that speed for at least 60 seconds. In order for a squall line form a squall must be accompanied by a severe weather storm most commonly a group of organized thunderstorms. However snow storms also produce squall lines.
A squall line also needs two weather fronts of different temperatures and densities to combine such as a cold front and a warm front or a cold dry front and a wet dry front.
The coldest of the two pushes then warmer air up producing updrafts, several updrafts can be present through out the squall line the more updrafts the line has the more severe the storm associated with it will be.
Depending on the most dominate factor of the squall line whether it be the wind or the weather front will be the determining factor of the severity of the storm and which of the two will be more pronounced either the wind or the thunderstorms that the combination of the updrafts, fronts and wind produce.
If the wind factor is dominate then higher than normal storm winds will be present with higher wind gusts. If the fronts and updrafts are more dominate than you will see more perception as a result of the squall line.
Often you will get an even mixture of both wind and severe weather which can produce damage from the high winds and flooding from the rain. As with any severe thunderstorm power outages also pose a problem with squall line storms.
It is also common for tornadoes to be spawn from these storms especially if the squall line is part of a supercell system or a tropical cyclone (hurricane, cyclone or typhoon.)
A squall line that is dominated by wind will usually dissipate quicker than one that is dominated by the weather fronts. A wind squall line is shorter lived because of the speed the wind is traveling.
Snow squall lines also known as lake-effect snow, are an increase in the wind speed that bring a sudden furious snow storm that only last a short time and due to the wind speed usually you will not even notice an increase in the amount of snow as the wind blows the snow around so quickly it creates more of a snow drift effect than an accumulation of snow.
Sources
http://www.theweatherprediction.com/habyhints/150/
http://www.atmos.ucla.edu/~fovell/ASres/ftan/ftan.html
http://www.vets.ucar.edu/vg/MCV/index.shtml
