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It seems that both these terms get thrown around a lot. Both, I think, describe the same idea. Which was established first?
Also, it seems like some people describe it as a good thing to have a low x factor, while others describe it as a good thing to have a high x factor. Which is it?
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Definition. The "bus factor" is the minimum number of team members that have to suddenly disappear from a project before the project stalls due to lack of knowledgeable or competent personnel. The expression "hit by a bus" describes a person either dying or more generally disappearing suddenly from the project.
The simple explanation for calculating a project or team's bus factor is to say: “What is the minimum number of people that, if absent, would prevent work from continuing?” If a project cannot continue because a specific person is unavailable, you have a bus factor of 1.
Good communication within a team ensures that information is shared, and further reduces bus factor. Practices like daily stand-ups, as well as frequently switching which other team members individuals work with (whether pairing or not), can help ensure knowledge is shared among all team members.
A high bus factor means that there are several individuals who know enough to successfully complete a project if an adverse event were to occur, such as a colleague falling ill, resigning, or going on maternity leave. Teamwork increases the bus factor for any project, and therefore reduces risk.
You want a high truck/bus factor:
Truck Factor (definition): "The number of people on your team who have to be hit with a truck before the project is in serious trouble"
(From: http://www.agileadvice.com/archives/2005/05/truck_factor.html)
i.e. you don't want parts of the code that only one person knows how it works or only one person can extend/maintain. Knowledge should be spread amongst the whole team via things like wiki info and pair-programming.
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Wikipedia says bus number is "more commonly known as truck number" But in the US, "hit by a bus" is practically an idiom, while "hit by a truck" is not (although either phrase is easily understood.) Regarding high/low being good, the wikipedia article says:
"High bus numbers are good (with the best case being equal to the number of developers on a project). This means all developers understand the codebase more or less equally. A low bus number (especially, one) represents a high risk."
I'd add to what @cartoonfox said: Promiscuous pair progamming is a good way to distribute critical knowledge around a team so that the truck number is as high as possible. If you don't swap pairs often and with many different team members, knowledge isn't distributed very quickly.
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