1. ARP is a protocol used with TCP/IP to resolve addresses. The TCP/IP stack used to transmit data across the Internet is independent from the Ethernet used to shuttle data between local machines. Thus, when machine needs to send an IP packet to a nearby machine, it broadcasts the IP address on the local Ethernet asking for the corresponding Ethernet address. The machine who owns the address responds, at which point the IP packet in question is sent to that Ethernet address. Key point: By sniffing ARP packets off the wire, you can discover a lot of stuff going on. This is especially true of cable-modem and DSL segments. Since ARP packets are broadcasts, you aren't technically breaking your user's agreement by sniffing. Key point: You can spoof ARP requests and/or responses in order to redirect traffic through your machine. From Hacking-Lexicon

2. a TCP/IP protocol used to obtain a station's physical address from its logical IP address. ARP requests are broadcast onto the network. From Linux Guide @FirstLinux

3. Linux ARP kernel module. From whatis

4. manipulate the system ARP cache From whatis