I've always heard the following: heavy weight + low reps = builds bulk. lighter weight + high reps = toned/cut.
Low reps, meaning less than 6, primarily
Reps between 6 and 12-20 (depending on muscle group) primarily
More than 25 reps build endurance.
You can't get definition in a muscle by doing more reps. Definition is achieved by overall reduction of bodyfat (and getting a thin skin).
Is it a good idea to switch off on weeks of doing heavy weight work outs/light weight workouts?
Yes, that's a good idea
Depending on your goals - for mass and strength, i'd say 4 weeks 8-12 reps, 4 weeks 1-6 reps, 2 weeks 12-25 reps. Repeat
How many reps and how much weight is good for toning/cut?
When you want to cut, i recommend using heavy weights (at most 10 reps) to prevent atrophy of the muscles while having a slight caloric deficit. Keep your protein intake up as usual, don't cut calories on protein. You can achieve the caloric deficit by eating less or working out more, either cardio or weights.