Cross-training in sports and fitness refers to the combining of exercises to work various parts of the body. Often one particular activity works certain muscle groups, but not others; cross-training aims to eliminate this.
For example a weekly cross-training plan would include a different workout for each of your 5 days of exercise. When you plan your workouts think about changing at least one component. For example: Time, distance, reps, sets, activity, weight load etc.
Cross-training helps to stave off boredom and encourages you to continue working out, the variety-themed programming aids muscles to adapt more quickly to new activies, and you may reduce your risk of injury by not over-using the same muscles repeatedly.
One big advantage of cross-trainng is it may enhance weight-loss efforts. :)