if I were to lift a certain exercise for 3 sets of 6 reps and then the next time I lift that bodypart I lift 3 sets of 10 but the weight is a little lower , is that still progressive overload? Assume that I'm going 1 rep before failure.
In this case you are overloading volume not intensity (weight on the bar).
The key with progressive overload is simply more stimulus over time, don't get too chaotic in your overloading and remember if you are training correctly then the overload can be relatively mild.
For example,
Let's say you can dumbbell press 70lbs for 10 clean reps but can't quite get 14 reps, especially not with good form. Your average workload is 2 sets before you see the total reps going down. For example,
Dumbbell Press (primary movement, all reps performed cleanly)
1x10 Easy
1x10 (arghh)
1x8 (from here its all down hill)
Now a progressive overload could be done in a lot of ways but I am a fan of simplicity, I would do the SAME weight until you can do 3x10 even if your 3rd set is SUPER challenging, but you do get all 10 reps.
Now I would add 5lbs...
You have effectively overloaded. Now continue to work until you can do 3 sets to completion.
Now lets say you continue on this track and you get to 85lbs and you hit a sticking point where you just can't make it to the 90s and its been 2-3 weeks of training smart.
Now I would go back to 70lbs and shoot for 4 sets of 12 and then go up to 75lbs once you are able to do all 4 sets of 12. Continue that track and then by the time you get to the 90s you can do 3x10 more than likely.
This is a very simplistic but effective approach.