Although much of your muscle strengthening and definition will come with typical weight lifting or resistance exercises, if you want your muscle definition to show or your muscles to feel hard, you'll need to include regular cardio.  Cardio exercises are great for a variety of reasons.  They help improve your mood and sleep habits, decrease your risk for obesity, diabetes, and high blood pressure, and can even help you have more efficient circulation.  In addition to the health benefits, regular cardio is necessary to help your body burn calories and decrease the excess fat that can accumulate both on top of and underneath your muscles.  The lower amounts of fat you have or the lower your body-fat percentage is, the more defined and hard your muscles will look and feel. Include at least 150 minutes of moderate-intensity cardio each week.  This could include:  jogging/running, rowing, dancing, using the elliptical, or doing an aerobics class. To get the best workout and get the best results, you'll want to do more than one type of exercise per muscle group.  Doing a variety of exercises can help ensure you're strengthening and defining your muscles in different ways.  Each exercise may work different parts of the muscle group or engage a variety of different smaller muscles.  This will help you get the most defined look.  For example, do not just do squats to tone your legs.  Do a combination of squats, lunges, calf raises, leg raises, and leg curls.  Each exercise is unique in which muscles they engage. Both higher reps and lower rep exercise plans can benefit your muscles in a variety of ways. Include both to get the best workout.   If you're doing higher reps with a lower weight, this can help you build bigger muscles.  These will show more, be more defined and may feel harder.  This is a good thing to keep in mind if your goal is also be to bulk up.  However, higher reps with a lower weight doesn't necessarily build strength.  It mainly builds muscular bulk.  Lower reps with higher weight help you build more strength.  The lower repetition amount and higher weights works your muscles differently that results in stronger more powerful muscles. Rest days are equally as important as days where you work out.  Studies have shown that the actual strengthening and size increase happens during rest, not work.   It's typically recommended to include at least one to two rest days weekly.  However, these should be "active rest days."  That means, instead of lounging around all day, do some light intensity or stretching exercises like walking or yoga.  In addition to your active rest days, make sure to rest between muscle groups.  For example, if you focus on lower body muscles on Monday, use Tuesday to work your upper body instead.

Summary:
Start with cardio. Include a variety of strengthening exercises for each muscle group. Do a combination of high repetitions and low weights and low repetitions and higher weights. Always include one to two rest days.