All undeveloped film changes over time. However, after exposure, films change much more rapidly than they do while unexposed.
Exposure to light converts silver-halide crystals into metallic silver, and the reaction continues after exposure, spreading slowly to nearby unexposed crystals in a kind of chain reaction, creating a uniform background density known as "fog." Contrast loss and color distortion can be apparent in as little as a few weeks, and quite noticeable in a year.
Films can be refrigerated or frozen without harm, if they are protected from humidity.
If your exposed films cannot be developed within about a month of exposure, you should store them in sealed zip-lock type plastic bags or Tupperware-type containers in your refrigerator or freezer.