StackOverflow = Programming-related questions - things about programming languages, algorithsm, data structures, implementation details, program design, et cetera - if it's involved in making software, it probably goes there.
ServerFault = System administration. OS setup/usage/sysoping, network management, et cetera all go here. If it's about installing/configuring production software/systems, it probably goes there.
SuperUser = General computer usage of all types. If it's a question about computing and it doesn't go on SO or SF, it can probably be asked here.
Meta = Questions about the other sites - support issues, questions on community topics, etc; anything that deals with how SO, SF, and SU work.