They are Pokemon #23 and #24. Both resembling snakes in appearance and characteristics (for example, they both hiss, have venomous/wrapping attacks, etc), Ekans is a boa (or a rattlesnake) while Arbok is a cobra.
In the TV show, they are a fairly main Pokemon. The main villianess, Jessie, has an Ekans, which later evolves into an Arbok. She got the snake for her birthday some time ago.
Ekans/Arbok is often called out to do battle against Ash and his Pokemon, or against whoever else is trying to stop Team Rocket completing their evil plans.
However, although it fights for the bad guys, it is not really a bad Pokemon. Occasionally it will stand against what it knows is wrong. It is strongly dedicated to its master, who although is nasty to most people and Pokemon, has a soft spot for her own Pokemon.
Arbok obeys Jessie even when it is at a fairly high level, despite that Jessie has no badges at all. So it must respect/like her a lot.
Arbok manages to become quite a strong Pokemon despite losing all the time. Perhaps it has had more experience than we are led to believe. It loses almost all the time because it is the villian's Pokemon, and the villians aren't allowed to win in a kid's show like Pokemon.
Arbok is around for a lot longer in the series than Ekans. Ekans evolves in episode 30 when it realises how much Jessie cares for it even if it's not very strong. So it evolves to please her.
No Ekans are shown in the TV series thus far with exception of Jessie's own. One other Arbok does appear in episode 51 though.
This website is mostly for either the game version of Ekans/Arbok, or for Jessie's Ekans/Arbok in the show.
Ekans is also a fairly common sight in the red version of the Pokemon game, and some Arbok can be found in the wild also.