Once in the country, the validity period of a visa or authorised stay can often be extended for a fee at the discretion of immigration authorities. Overstaying a period of authorised stay given by the immigration officers is considered illegal immigration even if the visa validity period isn't over (i.e., for multiple entry visas) and a form of being "out of status" and the offender may be fined, prosecuted, deported, or even blacklisted from entering the country again.
A visa is a stamp, sticker, or electronic record sitting inside your passport book that verifies that you’re allowed to stay in a specific country for a certain amount of time. They specify the length of your stay, what territories you may visit, your scheduled date of entry, how many times you may enter the country, and whether or not you’re allowed to study or work during your trip. Not all countries and territories require visas, but it’s best to stay up-to-date on regulations and requirements by doing your research and working with a travel agent. Immigration officials can revoke your visa at any time, and it’s important to remember that they never truly guarantee entry, especially in countries where visas are separate from formal entry permission. An official will likely review your circumstances once you arrive to determine whether or not you may enter.