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[carpetsmoker]

So it's just the way they're communicating, not the transferring speed...[/b]

yes, it is the way they are communicating.
This DOES affect your transfer speed (among other things of course).

Simplex is one way traffic, you can either only receive data, or only send data.
An example is Cable TV, you can receive data and watch TV, but you can't send data back to the network.
On the other side the network (let's say ZDF) can only send data to you, ZDF can't receive data from your TV.
Note: I am aware that there is digital television, but I think you get my point.
Using a 100Mbit NIC, your network speed would be ~95mbit (but only one way!)

You can more or less forget this method of communication where computers are concerned, it's practically never used.

(funny story: a friend of me broke his CAT5 cable, the idiot just knotted all four wires into one big knot, amazingly he still had internet! it was very slow tough ....3Com NIC's are the best! its transparently switched to simplex!)

Half-Duplex provides communication in both directions, you can send and receive data, but not simultaneously!
So, if I'm sending data to you, you have to wait until I've finished sending data before you can send a reply, and vice versa.
An example is radio (walkie-talkie's), if more than two people talk at the same time you'll get only static.

Again, this isn't used allot in computers nowadays, some peripherals use it (bluetooth, certain Parallel cable devices)

Using a 100Mbit NIC, your network speed would be ~45Mbit avarage.
You are able to send of data at ~100Mbit speeds, and recieve it at ~100Mbit to, but because of all the waiting the average speed will be about ~45Mbit.

Full duplex means that both computers can send and receive data at the same time, they don't have to wait for each other to finish.
This is what you're probably using, and how most of today's networks use.
Using a 100Mbit NIC, your network speed would be ~100Mbit

Half-Duplex is always slower, since only one computer can send at the same time, you might want to compare it to talking:
I say something for 5 seconds, and after I'm finished you say something for 3 seconds, total talking time: 8 seconds. (This would be "half-duplex")

Now we both start talking at the same time, we'll be finished sooner because we don't have to wait for each other (total time is now 5 seconds), this would be full-duplex.

And if I would do all the talking and you wouldn't say anything we would be communicating in simplex

Also, the receiving computer will have to wait until they're sure that the sending computer stopped sending data, which will slow these even further down (This is called turnaround time)
You can compare this to your reaction time in the talking example above.

On Unix-like systems (linux, BSDs, solaris) you can view your connection type with ifconfig
Example (bold line, ignore the SIMPLEX in flags):

# ifconfig -a
xl0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
options=9<RXCSUM,VLAN_MTU>
inet 192.168.100.13 netmask 0xffffff00 broadcast 192.168.100.255
ether 00:04:75:f3:b0:64
media: Ethernet autoselect (100baseTX <full-duplex>)
status: active[/b]

I have no idea how to look this up in Windows (ipconfig /a perhaps....?)

If you're really interested in data communication, then you might want to consider buying the TCP/IP bible, a very good book which covers all the important subjects!