M.
A., what you & your son are going through is *very* common - toddlers understand a LOT, but often they are not able to express their needs and/or their feelings (they don't have the language abilities yet - some do, but most don't). I really like this quote by Rue Kream about tantrums:
"To begin with, I don't think of them as tantrums, which to me is a word with a heavily negative connotation. Instead I think of them as times when my child is expressing how she feels in the best way she knows how. By assuming that my children are always doing the best they are able to at any given time, I am able to approach each situation as their ally."
One of the best tools I've found for tantrums is to try and help my kids express themselves, whether with words or signs. Both of my kids (and we parents, too!) learned a ton from the DVD series called SIGNING TIME (and I think it's even on PBS now, too, for free). It's a fantastic ASL (American Sign Language) show just for toddlers & kids, and I've never met a child (or parent) who didn't love it. Learning a few ASL signs together as a family has been *so* incredibly helpful for us, has reduced tantrums, has benefitted my kids verbally (earlier verbal speech, actually!), and is a wonderful bridge for toddlers when they're making the transition between limited speech and fully talking (preschool aged).
ASL is great, but it's not the silver bullet -- here's what else we do with our kids - don't know if it would work for you and yours, so disregard if it doesn't seem helpful for you:
This is really basic, but the first thing I try to do when my toddler is frustrated is to go through this quick checklist of needs (HUNGRY? TIRED? OVERSTIMULATED? EMOTIONAL NEEDS?)...
1) Is he hungry? When hungry, many kids (and even adults!) get less able to control their impulses, get grumpy, etc.
2) Is he tired or overtired? Again, when tired, impulse control (which is already a challenge for toddlers even when they're at their best!) goes way down. It may be time to shift into a quieter gear, maybe some hugs on mom's lap with a nice book, etc.
3) Is he overstimulated? If around too many other kids or in a place that's too busy or loud, toddlers can easily become overwhelmed/overstimulated - throwing/hitting is signal from the child to the parents "hey, mom and dad, I'm not able to handle this environment anymore - please save me/take me out of here!"
4) Is he emotionally needing reconnection? Toddlers are definitely vying for their independence, but they're also still babies! They need frequent check-ins with their parents, even if only for a minute or two, whether physical (hug or kiss or rub on the back) or verbal (I love you, sweetie!). If they're already tantruming, labelling their emotions FOR THEM OUT LOUD will often help them stop freaking out AND it helps them learn the words to what they're feeling, and eventually they'll use the words instead of screaming. This might look like: "Honey, I know you want to play with that knife. You really really wanted to see it, didn't you? Mommy is keeping you safe - you can't play with the knife, it is sharp, ouchie for you..." (etc.) We'll also use ASL here (the sign for "hurt" or the sign for "want" or the sign for "angry").
Anyway... that was a long way of saying:
* always check in w/ basic needs (emotions/empathy, hunger, tired, overstimulated), because if those are met, there are rarely tantrums
* try to give your toddler the tools to express himself better, whether through labelling (for him, out loud) his emotions during his tantrums, and/or using ASL signs until his vocabulary/verbal skills take over
Some great links:
Taming Toddler Tantrums
http://www.askdrsears.com/html/6/t063300.asp
Online, free dictionary of ASL signs (video!)
http://commtechlab.msu.edu/sites/aslweb/browser.htm
Signing Time - Public Television viewing schedule (so you can see it for free, though check w/ your public library to see if they have the DVDs to check out)
http://www.signingtime.com/pressroom/stationcarriage.htm
Temper Tantrums (by Rue Kream - the whole article)
http://www.naturalchild.org/guest/rue_kream3.html
Hope some part of this helps, and hang in there - this is only a phase, it will not last forever! :)