Configuration memory
Beginner
The many tiny memory cells inside an FPGA that hold its design: what each table says and which switches are on.
Novice
The memory cells spread across an FPGA that hold every LUT bit and switch setting. In most FPGAs they are SRAM, which forgets when the power goes off; some FPGAs use flash or one-time antifuses instead.
Expert
Distributed SRAM (or flash, or antifuse) cells whose outputs drive LUT masks, mux selects and block modes. Most bits control routing. SRAM configuration can be upset by radiation, mostly in routing bits.
Explained in Lookup tables and the FPGA fabric (Architectures).
See also: Bitstream, SRAM (static RAM), Flash memory.