Manual:Interface/EoIP - MikroTik Wiki
Ethernet over IP (EoIP) Tunneling is a MikroTik RouterOS protocol that creates an Ethernet tunnel between two routers on top of an IP connection. The EoIP tunnel may run over IPIP tunnel, PPTP tunne or any other connection capable of transporting IP.
When the bridging function of the router is enabled, all Ethernet traffic (all Ethernet protocols) will be bridged just as if there where a physical Ethernet interface and cable between the two routers (with bridging enabled). This protocol makes multiple network schemes possible.

Network setups with EoIP interfaces:
  • Possibility to bridge LANs over the Internet
  • Possibility to bridge LANs over encrypted tunnels
  • Possibility to bridge LANs over 802.11b 'ad-hoc' wireless networks

The EoIP protocol encapsulates Ethernet frames in GRE (IP protocol number 47) packets (just like PPTP) and sends them to the remote side of the EoIP tunnel.
Mikrotik RouterOS transparent bridge using PPtP and EoIP | Butch Evans Blog
EoIP Routing - DD-WRT Wiki
Extend your LAN between sites using an Ethernet over IP (EoIP) bridge and PPtP « Dale's Blog