Soreness

If you are active, you will eventually experience muscle tightness and/or soreness. It usually starts the day after fitness that has pushed past your threshold. It just means that your muscles are working to adapt to the new level of activity. It has an official name of Delayed Onset Muscle Soreness (DOMS) but it's not nearly as serious as that. If you're new to fitness, this soreness may seem like a bad thing, but if you can move and go about your normal daily life, it's nothing to worry about. It occurs to everyone, including seasoned athletes. There are ways to lessen and/or prevent this, but if that's forgotten or doesn't eliminate the problem, you can help yourself out after it has set in.

To prevent DOMS:
Take a cold shower or bath right after the exercise. This may sound awful and counter-intuitive, but it will really help prevent soreness.
Make sure you're hydrating and sleeping enough. These are key to helping your body heal.

To ease symptoms of DOMS:
You can gently work the muscles that are sore. If you are sore from running, try a short walk or bike ride.
You can stretch the sore muscles. This may remove the tightness and help the discomfort.
You can take a mild painkiller, but only if you can't deal with the pain. They delay healing, so will actually prolong the DOMS.