What makes a blizzard different from an ordinary snowstorm?

There are three criteria a snow storm must reach to become an official blizzard.

Glancing out the window at a raging snowstorm may make you say, "wow, it's a blizzard out there!" but it might not be a blizzard unless three crucial criteria are met.

First of all, it has to be snowing hard enough to reduce the visibility to less than 1/4 of a mile. That's just over 1,000 feet or roughly three football fields. 

Second, the wind has not to be just blowing but howling. The National Weather Service doesn't consider it an official blizzard until wind gusts reach at least 35 mph.

Finally, you need the first two criteria to last or expect to last three hours or longer.

Put all three together, and you have an official blizzard. Temperature and snowfall amounts are not considered in the definition, but you can surmise that both will also be a significant factor in any blizzard. 

If it's even close to freezing and you add in 35 mph wind gusts, you're talking at bare minimum wind chills in the low teens. Most blizzards will be colder and windier with even lower wind chills.

Heavy snow combined with strong wind will also create large snowdrifts that can bury wind-exposed landscape parts under feet of snow.  The great blizzard of 1966 reportedly had snowdrifts of 30-40 feet in the Northern Plains leading to this incredible photo taken in North Dakota:

And intense blizzards that bring near whiteout conditions can make travel extremely treacherous. Those caught outside, especially in rugged, mountainous terrain, can become disoriented and lost.