Here are two marks from the Qianlong period. One in regular script and one "seal" mark.
Imperial reign marks are like all Chinese texts read from top to bottom and from right to left. The first character is thus the one at top right as in the figure below. The marks are also written in one, two or three columns or rows. If it is written all in one horizontal row, and not in a museum, it is most probably a fake since this is an early Ming feature. Then we are talking tens of thousands of dollars.
During the Kangxi ( 1662 - 1722 ) period, marks with symbols and characters other than the reign title became common. The characters are often the name of the place the pieces seems to have been made for. These are called "Hall marks". It is also interesting to remember that specifically 18th century export porcelain to the West is almost never marked, while most pieces made for the Chinese commmon prople are actually quite often marked. All this commoners's porcelain is called Min yaw meaning "peoples wares" as opposed to the Imperial wares which is called Guan yao.
Regarding genuine marks it is the handwriting and several other small details together with teh glaze, shape, decoration, and other technical distinctions of a piece of porcelain which makes it possible at all to assign a piece to its correct period. This is VERY difficult and marks should be the last thing to consider when trying to determine a piece's authenticity.