Plank is exactly what he sounds like, a solid 2x4. Plank is always seen with Johnny, his creator and best friend. Plank can't actually talk but the show hints that he can communicate with Johnny, though this is most likely due to the fact that Johnny has an incredible imagination. Plank has two eyes that are uneven and a mouth, these were drawn on by Johnny.
In one episode it was hinted that Plank has parents as when Ed yelled that his parents were looking for him during the Hide and Seek game Plank poked his head out of his hiding spot.
During another episode Plank was possibly hypnotized, Eddy used a hypnotic disc found in Edd's Psychology Manual to hypnotize various kids in the cul de sac. When it was used on Johnny nothing happened, but then Johnny began screaming that he had turned Plank into a mindless zombie.
Plank also appears the be the "brains" between himself and Plank. When the Ed's asked Johnny for help with Ed's imaginary friend Johnny didn't know what that was, though Plank seemed to explain it to him.
Plank is also an official member of the Urban Rangers along with Johnny, Jimmy, and Rolf.
There was an instance in which Johnny and Plank had gotten into a fight and separated. During this time Jimmy took Plank under his care and treated him the same way Johnny used to, though by the end of the episode Johnny and Plank were reunited.
Whenever plank is lost, stolen, or could be damaged Johnny becomes incredibly aggressive and violent. An example of this behavior is in the first episode of the show when the Ed's begin dripping water on Plank, the sound and sight of this sends Johnny into a frenzy. There was another instance in which the Kanker Sisters stole Plank and used him as a massage post. Johnny eventually saved Plank from the Kanker Sisters.
Often times Plank is used by Johnny as a weapon, such as when he used Plank to tear apart the sails on the Kanker Sisters raft when they were after the Ed's, Jimmy, and Johnny. Plank was also used by the Kanker Sisters as a Plank after they captured the Ed's, Jimmy, and Johnny and forced them overboard.
Throughout the show Plank becomes a Superhero Sidekick named Splinter the Wonderwood. When Plank is Splinter he wears a simple mask to cover his eyes and is attached to the end of a broom. Johnny uses Spintler as a mode of transportation.
After the events of the Big Ed Picture Show Plank becomes a super villain named Timber the Dark Shard, though Plank then informs Johnny ( who becomes a villain called The Gord ) that the show is over, breaking the 4th wall.