A link is a layer 2 network packet transport medium, examples are Ethernet, Token Ring, PPP, SLIP, ATM, ISDN, Frame Relay,...
A node is a host or a router.
Generally a single homed host on a link. Normally it has only one active network interface, e.g. Ethernet or (not and) PPP.
A dual homed host is a node with two network (physical or virtual) interfaces on two different links, but does not forward any packets between the interfaces.
A router is a node with two or more network (physical or virtual) interfaces, capable of forwarding packets between the interfaces.
A tunnel is typically a point-to-point connection over which packets are exchanged which carry the data of another protocol, e.g. an IPv6-in-IPv4 tunnel.
Network Interface Card
The special character "ΒΌ" is used for signaling that this code line is wrapped for better viewing in PDF and PS files.
In generic examples you will sometimes find the following:
<myipaddress>
For real use on your system command line or in scripts this has to be replaced with relevant content (removing the < and > of course), the result would be e.g.
1.2.3.4
Commands executable as non-root user begin with $, e.g.
$ whoami
Commands executable as root user begin with #, e.g.
# whoami