From DD-WRT Wiki
In this tutorial, we will discuss different alternatives for linking routers in order to create wifi networks or just repeaters.
These are various ways that can sometimes be used to connect a router to another router or to a modem.
 Access Point / Switch
Extend the Wireless access area using more routers, with WIRED connections between routers, or turn a wired port on an existing network into a Wireless Access Point. All computers will be on the same network segment, and will be able to see one another in Windows Network. This works with all devices with LAN ports, and does not requIre ddwrt to be installed.
- Wireless Access Point - Extend Wi-Fi & LAN (Requires physical ethernet connection between routers)
- Switch - Similar config as WAP, but radio disabled (accepts only wired connections)
 Repeater / Repeater Bridge
Extend the Wireless access area using a second router WIRELESSLY connected to the primary. The secondary router must have ddwrt installed; the primary does not need ddwrt.
- Repeater Bridge - A wireless repeater with DHCP & NAT disabled, clients on same subnet as host AP (primary router). That is, all computers can see one another in Windows Network.
- Repeater - A wireless repeater with DHCP & NAT enabled, clients on different subnet from host AP (primary router). Computers connected to one router can not see computers connected to other routers in Windows Network.
- Universal Wireless Repeater - Uses a program/script called AutoAP to keep a connection to the nearest/best host AP.
 Client / Client Bridge
Connect two wired networks using a WiFi link (WIRELESS connection between two routers). The secondary router must have ddwrt installed; the primary router does not need to have ddwrt.
- Client Bridged - Join two wired networks by two Wireless routers building a bridge. All computers can see one another in Windows Network.
- Client Mode - Join two wired networks by two Wireless routers (unbridged). Computers on one wired network can not see computers on other wired network in Windows Network.
Extend the Wireless access area using more routers connected WIRELESSLY. WDS is a mesh network. Routers must almost always have the SAME chipset type for WDS to work, and any non dd-wrt routers must be WDS compatible. Using identical routers is best, but not always niece essay if all devices have the same chipset types. (All Broadcom or all Atheros etc)
Extend the Wireless access area using more routers. Extra routers do not need any wired connections to each other. Use several ISP (Internet) connections. OLSR is a mesh network.