Calculus using more than the usual two variables x and y in a function. The most common multivariable function would have three variables x,y and z, with z being the dependent variable.
A nicely organized and detailed study of calculus of 3 or more variables topics. Includes a review of minimum and maximum problems and vectors. http://omega.albany.edu:8008/calc3/toc.html
An interactive tutorial of multivariable calculus. A lot of work with graphics. Requires the use of DPG programming to participate in the examples. http://www.dpgraph.com/MCExamples/MCExamples.html