Please define heavy weight / low reps

I keep reading "lift heavy" with low reps vs light weights and high reps. Would some of you please define what you consider "heavy" vs "light" and "low" rep vs "high" rep?

I was planning on choosing weight and reps as follows:
Weight: Heaviest weight while maintaining proper form to almost muscle failure. ( I do need to be able to function after workout)
Reps: 8-12 small muscle (such as arms), 10-15 large muscles (such as legs)
Sets: Need advice..... Traditionally 3 sets, but read years ago that 1 set gave 90% of the results as doing 3 sets.

If doing multiple sets, is it better to decrease weight on 2nd, 3rd set, and maintain the number of reps, or to keep the weight the same for all sets, but decrease the number of reps.

I am not looking necessarily to build bulk, much more interested in a compromise between strength and endurance.

I am currently doing Cardio (with interval training) and flexibility, but now need to look at adding strength and balance into the overall fitness routine.

I will be using a combonation of body weight, dumbbells and balance ball as training aids. I will be working out at home without anyone to "spot" so I do need to use some caution.

At the moment I am still considered obese, I need to lose about 50 lbs to get to a healthy body weight. I don't know my real body fat %, but using online calculator it's about 30%. I don't neccesarly beleive this to be real accurate, but at this point it's probably close enough. I don't need to know the exact % to know that I'm fat. The mirror shows that well enough.