Clouds are a natural phenomenon that is surprisingly diverse, despite how uniform they may seem at first. There are a lot of different kinds of clouds which don't appear that different at first glance, though if you take a few moments you'll quickly realize that today's clouds probably don't look at all similar to yesterday's. This article will focus on clouds that fall into the 'middle' spectrum of the cloud scale.
As with any other type of cloud, middle clouds are defined primarily by the level of their base (not their top, which can stretch as high into the atmosphere as it wants). Middle clouds are those that begin at roughly 6,500 feet or above, to a maximum height of over 20,000 feet. Some of these clouds can also classify as low level clouds, often due to the presence of an overabundance of water droplets or ice crystals that lead to precipitation. Because middle clouds appear fairly high in the troposphere their contents can range from water to ice, depending on the altitude and the overall weather/season of the day.
The first type of middle cloud is the altocumulus. Appearing in the sky as a series of small, often closely-clustered puffballs, altocumulus clouds are typically quite white in appearance and very light and flighty. Altocumulus clouds could almost be seen as 'developing' clouds in that they usually signify the development of heavier rain or storm clouds later on in the day.
The second type of middle cloud is the altostratus. Like low-flying stratus clouds altostratus clouds usually appear in long, unbroken bands that can cover the sky with white or grey, often covering the sun or rendering it somewhat mute. Like altocumulus clouds these formations typically herald incoming precipitation, though usually not thunderstorms.
The third type is a variation of the altostratus, the nimbostratus. These clouds, loaded down with precipitation, fly lower than most middle clouds and appear as darker versions of altostratus clouds. Depending on the season and the temperature of the atmosphere they can release either rain or snow, and the presence of precipitation is what differentiates nimbostratus from altostratus.
As with any types of clouds, whether they're low, middle or high, identification of middle clouds takes practice and reference back to experts and pictures of similar clouds. Altocumulus clouds, for example, are not always going to appear in the same small concentrations, and thus can be difficult to identify without some experience. Keep your eyes on the skies and soon enough you'll be able to ID each brand without even having to think about it.
Lloyd, Julie. A Pocket Guide to Weather. Parragon Books, 2007.
Wikipedia: Altostratus Clouds