I just noticed that rustup can install toolchains and targets completely separate of each other. For example here I have installed three targets but only one toolchain:
The toolchain is made up of all the parts that run on your current host: cargo, rustc, etc.
The target has the parts needed to compile for a specific target environment, which may be different than the host. This is primarily the compiled standard library, but may also include additional runtime objects needed to link executables for that target.