Q:

What's the difference of "./configure" option "--build", "--host" and "--target"?

The script ./configure accepts 3 options --build, --host and --target. I'm confusing their roles. What's the difference and semantics of them?

A:

As noted in this blog post and alluded to in the GCC Configure Terms, --target only applies when you are compiling toolchains.  When you are doing normal cross-compilation of a library or binary you use 
--build=the architecture of the build machine
--host=the architecture that you want the file to run on

However, when you are building toolchains, things can get more complicated.  I think that the following is correct (though I can't say I've ever manually compiled a cross-debugger):
Lets say that you have:

a powerpc build machine that you are going to do all compilation on
several embedded devices, with mips processors, which your code is going to run on
an x86 laptop that you are going to use for debugging these devices in the field   

You would configure and build your debugging server (eg gdbserver) to run on your embedded device with 
./configure --build=powerpc --host=mips

so that you could putty on to your embedded device and run "gdbserver :1234 a.out" to start debugging and listen on port 1234.
You would then build your debugging client (which connects to and controls the gdbserver) with 
./configure --build=powerpc --host=i686 --target=mips 

which you would copy to your x86 laptop so that in the field you could run "gdbclient embedded.device:1234" in order to debug your a.out program.
This all applies to compilers too for which you might want to look at the GCC link above or this section about the Canadian cross compile.
Also note that, in practice, you might not see build, host or target specified because, according to this Autoconf manual page, "target defaults to host, host to build, and build to the result of config.guess."
In a word, build the code on --build, run it on --host with --target architecture environment.

A:

Note: Argument --target makes sense only when building compiler (e.g. GCC). When running configure before building GCC:

--build: the machine you are building on
--host: the machine you are building for
--target: the machine that GCC will produce binary for

From the GCC documentation (Host/Target specific installation notes):

If build, host, and target are all the same, this is called a native. If build and host are the same but target is different, this is called a cross. If build, host, and target are all different this is called a canadian (for obscure reasons dealing with Canada's political party and the background of the person working on the build at that time). If host and target are the same, but build is different, you are using a cross-compiler to build a native for a different system. Some people call this a host-x-host, crossed native, or cross-built native. If build and target are the same, but host is different, you are using a cross compiler to build a cross compiler that produces code for the machine you're building on. This is rare, so there is no common way of describing it. There is a proposal to call this a crossback. 

