They're caterpillars, and you're likely having a dermatitis reaction to the fuzzy hairs on them. Some caterpillars do have stinging hairs, but these look more like gypsy moth caterpillars, which aren't venomous per se, but the hairs can still be irritating if you're sensitive to them. If they are gypsy moth caterpillars, they're also quite capable of significantly defoliating trees, so undesirable for multiple reasons.

There are a lot of fuzzy caterpillars, so they're not definitely gypsy moths, but that would be my guess.