It depends on the country's immigrations laws. In some countries you can be held in custody and deported. In other countries you will just be fined. Still in others you will be charged with immigrations fraud and could go to jail.
No matter which one of the above actions results from overstaying your visa, if you should apply for a visa for some other country at a later time, the visa overstay stamp that gets entered into your passport can result in other countries refusing to issue you a visa.
Cheers,
William James Woodward, EB Experts Team