We only see Bertha Jorkins once, in Professor Dumbledore's memories. At the time, she's about sixteen, and is complaining to Professor Dumbledore. She followed another student and his girlfriend into the greenhouses, and he hexed her for spying on him. Professor Dumbledore asks, quite reasonably, "But why, Bertha [...] why did you have to follow him in the first place?" (30.144). This tells us just about everything we need to know about Bertha Jorkins: she's incredibly nosy.
She's also nosy enough to go to Mr. Crouch's house on a hunch, believing that Barty Crouch, Jr. might be in hiding there. Mr. Crouch has to cast a memory charm on her to make her forget. And she also has the great misfortune of traveling to Albania, where she spots Peter Pettigrew (now known as Wormtail), who's supposed to be long dead. Wormtail tricks poor, nosy Bertha into going with him into the forest, where he turns her over to Voldemort. Voldemort strips Bertha Jorkins's mind to find out several key things: (a) Barty Crouch, Jr. is still alive, well (mostly), and faithful to Voldemort; (b) Mad-Eye Moody is going to be teaching at Hogwarts; and (c) the Triwizard Tournament is going to be held for the first time in a century this year. That's all Voldemort needs to know to start the ball rolling on the whole Moody/Barty Crouch, Jr. switch.
Harry first hears Bertha Jorkins's name mentioned in his dream of Frank Bryce's death. Her disappearance is also one of the things that tips Professor Dumbledore off to the rise of Voldemort. But the Ministry of Magic is characteristically slow to act. She works for Ludo Bagman, and yet he doesn't bother to look into her death until the press (i.e., Rita Skeeter) starts making a fuss. However, even though it goes largely unnoticed, Bertha Jorkins's disappearance is central to Voldemort's plans.