My general is Lord of Tresserhorn and i'm trying to understand if Stifle or Squelch will work to counter his triggered ability. Is the ability an activated ability? What exactly constitutes an activated ability?
Stifle will work but squelch won't. His ability is a triggered CITP ability. This is a similar trick as casting Phyrexian Dreadnaught then stifling his CITP trigger to have a nice 12/12 for 2;P
Your 10/4 for 4's ability is a triggered ability (the 'When...' is a good indicator its a trigger), not an activated ability. So you can Stifle him (or a Phyrexian Dreadnought) to avoid the drawback. Squelch is out though.
You can identify an activated ability by it requiring some sort of cost, usually with a colon somewhere. Like on Archivist, "Tap: Draw a card", the tap is the cost of activating the ability. Squelch is of course more fun to use on an opponent, and when the cost includes a sacrifice. Sphinx's Herald, for example, has mana, tapping, and 3 sacrifices as part of its cost to activate. Squelch that for hilarity- the cost is still paid (mana lost, creatures sacrificed), but they don't get the effect.
In general- triggered = when, activated = has a cost:
You can identify an activated ability by it requiring some sort of cost, usually with a colon somewhere. Like on Archivist, "Tap: Draw a card", the tap is the cost of activating the ability. Squelch is of course more fun to use on an opponent, and when the cost includes a sacrifice. Sphinx's Herald, for example, has mana, tapping, and 3 sacrifices as part of its cost to activate. Squelch that for hilarity- the cost is still paid (mana lost, creatures sacrificed), but they don't get the effect.
In general- triggered = when, activated = has a cost: