Originally, 3e only hinted that sorcery came from a bloodline, something 5e returned to. The basic concept is that a sorcerer has a "spark" of magic, whether that's a draconic blessing, fey blood, or exposure to wild magic.
A wizard has insight. They also have a kind of "spark" but it's more reliant on learning that luck or raw knack.
The warlock is a bit of both; the pact involves a kind of spell-learning, but they gain a "spark" that fuels their abilities. However, their abilities have been overtly different. The warlock tends to be more "eldritch," with blasts of raw energy, fey and fiend powers, and so forth, a lesser focus on standard spell magic. Their style is different; they have frequent, flamboyant uses of powers, whereas traditional spellcastering involves some expenditure of resources.
So, it's not a purely arbitrary distinction. Each has distinctions in aesthetics, themes, and mechanical abilities. The 3e sorcerer was the least departure, but still, you have an arcane spellcaster who doesn't really need high Intelligence and may or may not be interested in making magic items. That was pretty radical at the time, and also, in 3e, there were more "dead levels" for just about every class, so the emphasis was on style and progression rather than scads of unique abilities.
I kind of miss the "chaotic OR evil" requirement of warlocks, but like the LG paladin, it was a rule honored more in the breach than the observance. I miss the connection to "wild magic" and limitless, monster-like magic. I really wish the 5e warlock had been Intelligence based, both to make eccentric weirdos more viable, and to emphasize their expertise in pact magic and how they learned more abilities, they didn't simply receive them like a cleric through inspiration.
