"They're just trying to get their way." Sometimes, yes. But not always, and telling the two apart changes what actually helps.
What a tantrum looks like
A tantrum is usually goal-directed. Your child wants something (the toy, the extra episode, no bedtime) and is using big behavior to try to get it. In the middle of a tantrum, most kids are still tracking their audience: watching your reaction, checking if it's working, sometimes even pausing to see what you'll do next.
What a meltdown looks like
A meltdown is different. It's not aimed at getting something. It's what happens when a child's system is overwhelmed by noise, transitions, hunger, tiredness, or too much sensory input, with no "want" driving it. A child in a meltdown usually isn't tracking an audience or checking your reaction. They're not performing distress, and they're not able to stop it on command the way a tantrum sometimes can be redirected.
Why the response should differ
A tantrum often responds to calm, consistent limits: staying steady, not giving in to the demand, and letting it run its course without an audience.
A meltdown responds to a completely different approach: reducing input, not adding more talking or reasoning, and helping the body settle before anything else. Trying to reason with, redirect, or set a limit on a child mid-meltdown, the way you might with a tantrum, usually makes it worse. There's no thinking brain fully online to negotiate with yet.
When you're not sure which one it is
If you honestly can't tell which one it is, respond as if it's a meltdown: calm, low words, reduce the input, stay close. It's rarely the wrong move to offer regulation support even to a tantrum, and it's often the only thing that helps a true meltdown.