heal vs sustain
The act versus the property. A heal is a spell that restores health, now; sustain is the ability to recover health over time, by any means.
heal
Heal names one precise act: a healing spell, or the summoner spell of the same name. It has a caster, a target, a moment. "heal me" is asked of a support; "he has heal" is tracked like a flash, because it changes the maths of an all-in.
open the page →sustain
Sustain describes a capacity: holding a lane by recovering what you lose, through heals, lifesteal, regeneration, potions. It is not a spell, it is a total. "he has too much sustain" means trades are pointless, he gets it all back.
open the page →Why they get mixed up
A heal contributes to sustain, which is why they get mixed up. But they are not countered the same way: a heal is played around its cooldown, attacking once it is used; sustain is countered with anti-heal or by changing method, all-in instead of poke. Naming the right one gives the right answer.