During the first half of your contract - but with a maximum of 6 months - your employer can choose to terminate the contract on the condition the the employer respects the notice period agreed upon, if not then your employer will need to pay you a compensation.
After this first half (or 6 months) your employer can still terminate the contract before the expiry date but will need to pay you a compensation by paying your salary until the end of the contract.
For more information and examples: http://www.emploi.belgique.be/defaultTab.aspx?id=42270