Ionization is the physical process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions.
The process works slightly differently depending on whether an ion with a positive or a negative electric charge is being produced. A positively-charged ion is produced when an electron bonded to an atom (or molecule) absorbs enough energy to escape from the electric potential barrier that originally confined it, thus breaking the bond and freeing it to move. The amount of energy required is called the ionization potential. A negatively-charged ion is produced when a free electron collides with an atom and is subsequently caught inside the electric potential barrier, releasing any excess energy.
For more, visit the Wikipedia entry for aqueous solution, URL: http://en.wikipedia.org/wiki/Ionization