When atoms join in a covalent bond the share some of the electrons
in their outer shell.  
Notice that the hydrogen atom now has two
electrons in its outer shell (full) and the
oxygen has eight (also full).  This is
because the electrons in the outer shell are
shared between the three orbits of the
three atoms.
The simplest of all covalent bonds is
found in a diatomic molecule of
hydrogen.  In the picture shown, notice
that there are two electrons in orbit
around two atoms.  This makes both
atoms complete and less reactive.
Covalent Bonds - Sharing Electrons.