Payoff at expiry is max(S - K, 0). Losses are capped at the premium; gains are unbounded.
Why the asymmetry matters. That capped downside is why more volatility raises a call's value: extra upside is worth something, extra downside is not, because you simply do not exercise.
Early exercise. Never optimal for an American call on a non-dividend stock - selling always beats exercising, because you would forfeit positive time value and the interest on the strike. The one exception is just before an ex-dividend date.