crash test rider is right but so is tj and cookieaa. IS refers to the way in which the caliper or adaptor fits to the frame fork, its the International Standard, as opposed to the post mount method.
as a general rule, you can bolt an IS caliper straight on to an IS frame or fork if you are using a 160mm rotor. same for post mount frame and post mount caliper.
you can then use adaptors for both systems to run bigger rotors.