anyone can help me understand this terms:

-dhcp
-bootp
-vpn
-inetd

well i'm new in using linux and i don't have much friends to share about it
btw is it possible to connect to computers directly using ethernet card without any switch or hub or anything??

As for the definitions, I started but then realised it would take forever... sorry. Briefly, very briefly.
DHCP: Dynamic Host Configuration Protocol. Automatic IP address assignment to clients from a DHCP server eliminating the admin overhead/nightmare of assigning IP statically.
BOOTP: Another way of obtaining an IP address at boot-up.
VPN: Virtual Private Networking. Using shared/existing infrastructure (=internet in most cases) to create a "Virtual Private Network". In other words, despite using a public medium such as the internet, a virtual private network can be created between geographically disperse locations via encryption protocols such as IPSec.
inetd... no idea, noobie for Linux. I only know it has been replaced by xinetd???
And yes, it is possible to connect computers withoug a hub/switch/router.
I use this method, remembering that a router is essentially a dumb as pc, or a pc is a smart ass potential router.
Device A: Hub/Switch.
Device B: Router/PC
If you connect 2 like devices, you need a cross-over ethernet utp cable. If you connect 2 unlike devices, you need a straight-through cable.
So, for you case, to connect 2 pcs each with an ethernet card without a router/hub/switch you just need a cross-over ethernet cable... and correct IP assignment.
Hope I kind of helped?

xinetd (replaces inetd) - 'superserver': the program which is used for starting non-resident services.

Usually a service (best example: http) is opening the port 80 and waiting for requests - it deals with connections itself and is working permanently (it resides in memory and is running permanently)

The second method of starting services is using xinetd. You can add several services to the xinetd.conf. This way the only 1 service (i.e. xinetd) is running and listening on declared ports and when a connection call is made to a port it starts a relevant service.

BTW: if the service can be run directly or via xinetd depends on the service (since different communication methods)

Sorry to be a stickler for details (its just my nature, no flame intended) but the VPN connection is actually more of a function of a tunnelling protocol like L2TP or PPTP. Your packets (that you want to keep private) are encapsulated and routed using a tunnelling protocol, and then un-encapsulated at their destination. It is true that often times you will encrypt the connection to help with security, but IPsec by itself does not actually do the tunnelling.

slight