Its most common function is that of a lock-pick, but can be used to perform other operations such as conducting medical scans, remotely controlling other devices, providing orgasms, making the Doctor look like a complete fucking lunatic by waving it around all the bloody time, tracking alien life and, using red setting or dampers, it can control the properties of atoms and molecules on a small scale, create triple super beams that destroy Daleks, give the user telekinetic powers to lift massive stone objects, destroy guns, re-seal temporal rifts, resurrect people in the form of immortal faces on a pavement slab. It can, with the exception of a deadlock seal or wooden lock, open any type of lock and operate computers, whether their origin is alien or human. Try to distinguish the satire from the actual things the screwdriver has been used for, I fucking dare you, it's the everything power for whatever the scriptwriters need it to be.
Doctor Who spin-offs have replicated its functions in devices such as the sonic lipstick (for sonic kissing), sonic blaster (for sonic cake-frosting), sonic pen (for writing songs), and sonic modulator (for fucking sonics).
The sonic screwdriver was used regularly throughout the Second Doctor's tenure. It was used to turn screws and nothing more, a function which has been long since forgotten about. It became less prominent from the beginning of the Third Doctor's tenure because he was one bad motherfucker who didn't need a sonic dildo much. It was written out of the series in 1982 due to the limitations it caused when writing for the show, then made a full return in the 2005 continuation of the series, where it's been limiting the writing of the show ever since.
The Sonic Screwdriver is no more.
Throughout the programme, there have been many different versions of the sonic screwdriver.