AMPS: Amperage or Amps provided by an electrical service is the flow rate of "electrical current" that is accessible. Accurately, Amps = Watts / Volts. (Amps = Watts divided by Volts)
If you have a 100 Amp current flow rate available, you could, speaking very roughly, run ten 10 amp electric heaters concurrently. If you have only 60A available, you won't be able to run more than 6 such heaters without risk of overheating wiring, causing a fire, nimble a circuit breaker or blowing a fuse.
Ampacity, in the electrical code, refers to the current, measured in amperes, that a conductor (a wire) can carry constantly under the conditions of use without exceeding its temperature rating - in other words, the ampacity of a #14 gauge copper wire intended for residential electrical wiring is 15 Amps because that's the amount of current that the wire can carry without getting too hot. "Too hot" means a temperature that could harm the wire insulation and thus reduce its safety.
Volts: Volt, properly, is defined as the potential difference across a conductor when a current of one ampere dissipates one watt of power. This description is not very helpful to consumers. Using water-in-pipes analogy, volts is similar to water "pressure" in the electrical system. Having higher "pressure" in a pipe (or electrical conductor) means that conductor is able to delivering more energy to the user. Accurately Volts = Watts / Amps. (Volts equals Watts divided by Amps).
KVA: KVA stands for Kilo Volt Amps, which is calculated by a volt and amp meter. KVA is for clear power. For genuine Power, KV (Kilo watts) is used. It is used for sizing wiring, circuit breakers, and UPS (uninterruptible power supply).A volt-amp and a watt are not similar. kVA is used to compute what size wiring you need, what size breaker, etc, to power a circuit of a given amperage/wattage. KVA rating is higher than the wattage on an AC circuit.