By default the sshd service is not installed in the debian distrubution used on the Raspberry PI. For most uses this isn’t really necessary but if you are intending to use the pi without a screen then you need some way to get into it over the network.
All you need to do is to install ssh & tell debian to run it on boot:
sudo apt-get install ssh sudo update-rc.d ssh defaults
Now, as long as you know the IP address (or you give it a static one) then you can now ssh into it.