10BASE-T using MAC and PHY chips
Making a connection to an Ethernet network usually requires the use of a "MAC" and "PHY" chips (for "media-access controller" and "physical-Layer Interface").
The MAC connects to the PHY using a standard "MII" bus (Media Independent Interface), so different MACs and PHY can communicate together.
The project on this site doesn't use a MAC/MII/PHY but try instead to provide the minimum MAC/PHY functionality required for Ethernet inside the FPGA.
The MAC functionality is easy to emulate in an FPGA.
The PHY provides many services that can be easily implemented in an FPGA (serialization/Manchester encoding...).
It also provides some analog filtering ("wave-shaping"), since the Ethernet specification calls for specific wave shapes on the output.
See chapter 14.3 from the IEEE 802.3-200
A transformer usually follows the PHY (for electrical isolation).