Just as an air 'parcel' is a mathematically confined small batch of air suitable for rising and falling and condensing and convecting, so an air mass is representative of a much larger batch of air, indeed one that covers an entire section of the country.
    An air mass basically encompasses all the air over a region of the country that has basically the same properties of relative temperature and moisture. Air masses can be small enough to cover just a few states, or perhaps half the country. They all originate from some place, and they have some sort of similar temperature and moisture consistency to them. For most cases, they are easily classified and can be recognized easily on any surface map of the country.

Air Mass Classification

    On planet Earth, it's quite obvious that air hovering just over the surface can have one of two source regions: Over land or over water. An air mass that has over-land qualities (Relatively stable, not as much moisture, usually some extreme in temperature either hot or cold) is typically called Continental. An air mass that has the characteristics of forming over water, whether it moves many miles onshore or not (moisture-laden, humid, somewhat unstable, not dramatic in temperature), is a Maritime air mass. The second element in the mix is the latitudes at which the air forms, but a simpler way to remember it is thinking about the relative temperatures the air brings. If an air mass overhead is responsible for the temperatures being 10 degrees above normal, then it is typified by a Tropical rating, having originated in the tropical latitudes. If an air mass brings temperature that are 10 degrees below normal, then it is said to be Polar. Sometimes when it gets extremely cold, an Arctic air mass would show up, and by the same token, an Equatorial air mass would represent really hot weather. It's not easy to see air come straight up from the equator to New England (although it's somewhat simpler to see air come from the arctic on down), so often these are just classifications based on relative temperature (departures from normal, etc.).  The way air is classified is by putting the first letter of the source region first (lower case), so it would be either a "c" or a "m". Next would call for the capital letter of air in question, either being a 'T'ropical air mass or a 'P'olar air mass. Here is the breakdown of the air masses that typically exist over the US in a given year:

    These air mass characteristics are not necessarily permanent, for air can move and change as it interacts with the ground, or even other air masses. Moreover, the air masses have a slightly different look in the winter than the summer. Most notably, the "Continental" air masses, though mostly stable in winter, can develop some pretty good storms in the summertime provided a bit of surface heating-induced vertical motion. In the winter-time, modification of otherwise stable continental polar air due to warm lake water is a chief example of the "Lake Effect" snows that plague the Great Lakes states (not so stable then). Just for example...Let's see what we have on the weather maps today:

Current Temperature Map	Current Dewpoint Map

    Recall that the dewpoint is a pretty good indicator of the amount of moisture in the air, and the closer the dewpoint is to the actual surface temperature, the closer to saturation the air becomes. Putting those maps together, we can spot a few air masses with relative ease:

    You can see by the map of the US, there are a few air masses present, moving large amounts of air about. The warm, moist air in the Southeast is Maritime Tropical air and it's carrying lots of moisture up to the north, as indicated by the high dewpoints in that region. Likewise, cooler and drier Continental Polar air is filling into the plains states, as you can see cooler temperatures and lower dewpoints over there. The hot and dry conditions in the southwest is because of Continental Tropical air. Lastly, the Pacific Northwest is experiencing Maritime Polar air, signified by its cooler yet moist conditions. You can see the typical source regions for the most common air masses that travel through the US here:

    As you can see, the air masses will flow across the country and sometimes their directions of movement will put them on collision courses with one another. This is a fundamental meteorology and it all revolves around Fronts.

Fronts

    The air that's moving within an air mass is either expanding or pushing its rim into new territory. This outer rim is usually marked by what's called a Front, especially in the way that air interacts with the air or air mass out ahead of it. There are several types of fronts, and their names are typically dependent upon the air that they are advancing. One thing to always remember: The cold air rules. It, by virtue of the higher density colder air brings, is always on the bottom pushing around the warmer air. The three main types that appear are as follows:

   Cold Fronts are shown on maps by a blue line with carrot-ed edges pointing outward from the cold pool behind it. It is showing the advancement of cold air into a region of warmer air. Interaction along the edge of this front often results in showers and storms developing, as the warmer air is pushed upward by the advancing colder air (upward motion always helps get those clouds forming).
Warm Fronts are regions of warm air that are advancing on colder air. Since this air has to overrun some stubborn cold air, it generally doesn't produce severe thunderstorms, but rather the continual widespread but slow upward motion contributes to more prolonged precipitation. Behind the warm front, even though there may be a lot of moisture in the air, there is no lifting mechanism, so it's usually more quiet. Warm fronts on a map are red and have scalloped edges pointing towards the cool air it's attempting to override.

Stationary Fronts are pretty much just that: Stationary. Two air masses that are just quite slow to move sometimes will interact just along a fringe, and that line will have a few showers or even thunderstorms firing along it, but it's generally not moving anywhere or advancing any warmth or cold. The weather along these fronts are similar to warm fronts, less turbulent, but often longer lasting. Since there's no large scale advancement of warmer or cooler air, the stationary front is marked by alternating strokes of cold and warm front pieces. Often when an advancing cold front gets too elongated, the air mass that's behind it loses some of its characteristics, causing it to slow up and eventually just stay relatively...well... stationary.

Occluded Fronts are usually only seen in mature cyclones, and they develop right around the same time a storm system has peaked in strength and will begin to weaken. Here onrushing cold air, which is most dense and thus digs under all air with warmer temperature, has at last elevated all warm air ahead of it and has become separated from a warm influx and is left with cool air in its place. Warm air may still be fed into the system for energy, but it will be at points farther and farther away from the beginning of the occlusion. 

    On the same weather map this particular day, the cP air is interacting and advancing upon the mT air. In the summertime that is often a recipe for thunderstorms. Oddly enough this system set a new US record for a hailstone that fell in Nebraska (17"+ in circumference!). Since the advancing cold air is always dominant being the denser air, it is represented by a cold front. In the most stable air towards the middle and back of the advancing air mass, one often sees a High pressure point, so it's a good indicator of what air masses are present. Remember, air travels clockwise around High pressure centers, and counter-clockwise around the Low pressure centers.