Although you can use a Raspberry Pi's desktop using it's HDMI port and a monitor, you can also use VNC. To do this, you will need to install tightvncserver on your Pi, with this command:
sudo apt-get install tightvncserver
Next, you need to create a desktop to connect to on the Pi:
tightvncserver -geometry 1200x800 -depth 24 :1
This will create a new desktop on port 5901. You can connect to it with another computer using VNC viewing software. You just have to enter the Pi's IP address and the port. Something like 192.168.1.34:5901. You can find your Pi's IP address by using the
ifconfigcommand on the Pi and looking for inet addr:inet addr:192.168.1.34