novice inquiry; real number, jabber room


I am starting to learn fortran, reading 'fortran 90/95 explained'.

I do not understand the difference between a number '3' and '3.', i.e.
the significance of the full stop character. Any advice please?

Also, I created a jabber room, 'fortran@xxxxxxxxxxxxxxxxxxxxx'. I was
hoping to be able to communicate in real time.