Complaints means expression of displeasure against somebody or something.
Grievances means a formal form of complaint following due procedure against any authority or person. For example, if someone is aggrieved against any rules and regulations in a company he/she may give a formal complaint to grievance cell of that organisation maintaining the procedure.
When you suffer from some irregulariy or wrong doing, you feel aggrieved and hence you lodge your grievance. Expression of your grievance is also a complaint. Yiu may lodge a complaint even when you are not personally aggrieved. For example, there is traffic concentration at some place in the city, and there is no traffic police arrangemment, you may make a comlaint. Here is no grievance but a complaint.
Complaints are offensive in nature as the complainant expresses anger in order to express his feelings while grievance can be a request also as an aggrieved person puts forth his concerns and the motive is to get it resolved. :)
grievances are those matters which are not solved and the employees have to tolerate the vulnerable situations forcefully but they are totally against of it .complaints are those statements which are raised in order to avoid the vulnerable conditions.