If would be worth having a quick google on the difference between an Access Port and a Trunk Port, there are plenty of people who are a lot better at explaining things than I am :)

Generally a Trunk port is used between switches but can be used to connect a Switch with a Router or even a Switch with a Server. Both devices that you are connecting together need to be capable of 'tagging' packets before then send them across the link. 

Have a read up on Access and Trunk ports and then read up on Untagged and Tagged, if you can get round the concepts then you should understand L2 switching technology a bit better.

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A (Cisco) switch trunk allows you to send have multiple VLANs use the port.  (VLANs, excluding native, are tagged so switch can tell them apart.)

On routers, you create a similar port by defining subinterfaces, each subinterface defined to use a certain VLAN tag.

(router) e.g.:

interface FastEthernet0/0
 description to switch trunk
 no ip address

interface FastEthernet0/0.1
 description vlan 1
 encapsulation dot1Q 1 native

  ip address 192.168.1.1 255.255.255.0

interface FastEthernet0/0.45
 description vlan 45
 encapsulation dot1Q 45
 ip address 192.168.45.1 255.255.255.0