In this case, an Intellectual Property Block, or the Verilog/VHDL gate description of a specific subsystem:
Ethernet MAC 10/100 Mbps
I2C controller core
SD/MMC communication controller
USB 1.1 Host and Function IP core
SPI controller core
UART 16550 core
Most of them have a common 'bus-type' inside the chip; a lot of these open ones are Wishbone based.
See http://opencores.org/ for a list of many more IP Cores. There are thousands more proprietary cores one can license from a company, for example, ARM. Some charge more than others. Some won't allow you to synthesize designs; some will license it separately.
Most of them can be added to the bitstream of a FPGA, a Field Programmable Gate Array (Basically a big block of logic gates that can be programmed to 'be anything') and later, burned into silicon as an ASIC design, after the synthesized FPGA design proves itself in field testing.