Humidity can be thought of in two different ways; relative and absolute humidity. When used by itself, it almost always refers to Relative Humidity.
Absolute humidity is the absolute measure of the amount of water vapor in the air. Given a fixed amount of water vapor in a fixed amount of air, absolute humidity will not change (as long as there's no condensation) as the temperature of the air changes like relative humidity does.
Relative humidity is a relative measurement of the amount of water vapor in the air compared to how much the air can hold. Air can hold more water vapor the warmer it is, but its ability to hold water vapor decreases as the temperature goes down. Let's assume that the air is not saturated for this example. As the air temperature cools, the absolute humidity stays the same, but the relative humidity increases. If the temperature decreases enough, then the air will become saturated. The point at which the air becomes saturated is the dew point. At the dew point, the relative humidity is 100%.
The relative humidity increases as temperature decreases because the amount of water in the air is unchanged, but the air can hold less. On the other side, if the temperature rises, and the absolute humidity remains unchanged, then the relative humidity will go down. The air can hold more water when its warmer, so the relative amount it can hold increases.
So a relative humidity in Winter (colder temperatures) of 100% means that the air is saturated. A relative humidity in Summer (higher temperatures) of 100% also means that the air is saturated. But the tricky part is that given saturated air (100% relative humidity, or the air temperature equals the dew point temperature) in both seasons, the Summer air will have a higher absolute humidity (or more water) than the air in the Winter. It is this confusing relationship that prompts the general public to use relative humidity, because it is entirely relative to the temperature of the air, thus you know by looking at the percentage how saturated the air really is.

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