Yojimbo are the people running around con wearing the red sashes or red haori.
If you're not familiar with a haori, it's a kimono-type jacket with long "under-sleeves". It's
what Kenshin wears in Rurouni Kenshin, though most of the yojis wear something underneath. (Most. Sometimes, we have trouble even getting them to wear that much.
*cough kilts and towels cough*)
The yojimbo aren't required to wear sashes or haori, and sometimes they don't - especially when the temperature is a little high, you have to climb a lot of stairs, and the hotel's A/C doesn't seem to be working particularly well. They usually do, though, for the sake of being easily recognizable when someone needs help.
Translated, it means Bodyguard/Security, which is NOT actually what we are. We are a all volunteer group (As is the entire con staff), vary few of us are certified to do security work (Yes now a days you actually have to be certified by the state of Oregon to do bodyguard and security work), which is why we have private security and the hotel security around!
But, the name is cool.
This.
Just for reference, as LexManos says, staff is all-volunteer - our reward is seeing fellow con-goers have a great time and make wonderful memories, not the other way around. It may be hard to believe when they have to ask you not to do something, but the yojis really and truly
don't like enforcing silly rules or making people unhappy. (People who do don't belong in the yojimbo, and they don't stay in the yojimbo.)
The reason they're there is because someone has to enforce the rules, and it's worlds better when that someone is part of the anime fandom rather than someone who sees us as weirdos to be barely tolerated. I think almost anyone who went to Kumoricon 2009 knows how much of a difference that makes.
