Some of these pictures and descriptions may give away plot details that you might not want to know before watching the film.
This is a sci-fi police drama with a killer robot and, thankfully, no romantic subplot to get in the way of the action.
Denzel Washington is Parker Barnes, an ex-cop convicted of murder who is let out of prison temporarily to help train police recruits in a virtual reality simulation run by the Law Enforcement Technology Advancement Center. (The VR simulation is run on computers with heavy keyboards and huge CRT monitors, which look especially out-of-date now 20 years later even though the film takes place in the future.) Russel Crowe plays Sid.67, a sadistic, intelligent, and dangerous computer program made up of the personalities of more than 150 serial killers. With the help of Lindenmeyer, the corrupt programmer who developed him, Sid 6.7 is taken out of the computer program and turned into an almost indestructible nanotech synthetic organism android who runs around L.A. killing people just for fun. Parker Barnes is freed from prison and sent to hunt and destroy him.
After Sid 6.7 kills a trainee cop in the VR simulation, the LETAC bosses want to destroy him. Lindenmeyer decides to save Sid by turning him into an android. He convinces a developer who is working with nano technology androids that he will give him the sexy virtual reality woman he has developed so he can turn her into an android to do "research" on, but he actually brings him Sid 6.7 instead.
First, the nano tech guy wants to show Lindenmeyer how his nano stuff works, so he brings out an android snake he calls Elizabeth. She isn't a rattlesnake, but makes continual rattling noises anyway. He cuts off Elizabeth's tail with a scissors and finally the rattling stops and she makes a pained squealing sound. Then he puts her wound into a solution of nano machines which quickly re-generate her tail. Unfortunately the stupid rattling sounds also begin again. Then he pulls the character module out of the snake's mouth which produces "nano death" and Elizabeth goes limp. After Sid 6.7 becomes an android, he can also heal his own wounds and severed limbs by eating windshield glass or shoving his hand into a window because the nano cells are silicon based. I know what you're thinking. I don't buy all this science either, but this is what they want us to believe.
The snake is a special effect, that's most likely partly a prop he holds in his hand, and partly a CGI effect.