The only Gollum we've ever known is a big eyed, bony creature that crawls almost naked on four limbs. He's, um, not a looker. He has weathered skin and only a few wispy strands of hair—he's been made decrepit both from the Ring's corruption and from the fact he's been living pretty literally under a rock for the last five centuries.
But Gollum wasn't always this way. He was once a stoor (a hobbit-like being) named Sméagol, who found the Ring oh so long ago.
Living alone with the Ring, Sméagol has all but disappeared as he was slowly replaced by his Ring-obsessed alter-ego, Gollum (named because of a nasty cough he developed—when he hacks it sounds like he's saying "gollum, gollum."). There's no knowing what Sméagol was originally like, but we don't think he was crunching down on raw fish and talking with absurd pluralizations—both common side effects of possessing a ring of power. But then came Frodo who, by showing Gollum a mixture of compassion and pity, is able to partially resuscitate Sméagol and some of the basic humanity Gollum had long since lost.
Dissociative Identity Disorder and its Effects on Dietary Preferences and Pupil Dilation
But just because Sméagol is back doesn't mean Gollum's just gonna take a hike (although that's one way to describe their trip to Mordor). These two personalities duke it out in a battle of wits. Sméagol is grateful that he's finally met someone (Frodo) who cares about who he is. Sméagol wants to trust master and help master reach Mordor so he can destroy the… Ring?
This is where things get a little dicey. Sméagol may want to serve Frodo, who rescued him from that itchy rope that chokeses him, but Gollum definitely doesn't want to see the Ring thrown into the fiery pit of Mount Doom. Gollum will do whatever it takes to save his one, true master: the Precious. He has a lot of mean things to say about Sméagol: he's a liar, a thief, and even a murderer. We're not sure if these things are true, but when Gollum says, "You don't have any friends. Nobody likes you," we know he isn't telling the truth because Frodo does genuinely care for Sméagol.
Or so Sméagol thinks. Frodo, somewhat inadvertently, tricks Sméagol at the sacred pool and loses all of his trust. So in the final scene, Gollum is back once more and arguing with Sméagol. But this time, Sméagol isn't defending Frodo, he's simply too timid and scared to kill him and take the Ring back; "It's too risky." So when a mysterious she comes up, Sméagol immediately jumps on board Gollum's plan.
This is where Sméagol ceases to be a separate identity, and becomes merely a façade that Gollum can use to hide his true intentions from the hobbits, especially "the fat one." We realize just how connected these two sides of him are, and how silly it was of us and Frodo to think that a few weeks or months of minimal kindness (like not making him a slave…) could undo five hundred years of servitude to the dark lord.