The legends on Chinese coins can consist of up to nine characters. The most common is four characters, but a few coins have been issued with no legend at all, like for example the Ding 514. When there is only one character on a round coin, it is placed on the right hand side (Schjöth 73). When there are two characters, they are usually read from right to left. One exception is the Han Xing of the Jin dynasty (Ding 559) which is read from top to bottom.
Before the Tang, legends often included weight units and numbers. Sometimes there were also place names on the coin indicating which city or state had issued it. But the characters could also have other meanings, like the above mentioned Han Xing, which means "the rising of the Han dynasty". After the Tang, the vast majority of coin legends were reign titles followed by the two characters Tong Bao (circulating currency), Zhong Bao (heavy currency) or Yuan Bao (primary currency). Coins with four characters are usually read top-bottom-right-left, but in some cases top-right-bottom-left (e.g. Ding 796) and even top-bottom-left-right (e.g. Ding 827).

The obverse.

Reign titles.
In a few cases before and almost always after the Tang dynasty, coin legends included the name of the emperor. But emperors had many names, and they could be long, in the case of Nurhaci 29 characters (Wilkinson p. 109). Abbreviations were therefore obviously needed, especially for coins.

From the Shang to the Sui emperors were usually referred to by their posthumous title, Shihao, like Zhou Li Wang or Han Wu Di. It was Han Wu Di that was the first Chinese emperor who adopted a reign title, nianhao, in 110 B.C. (Wilkinson p. 182). With a few exceptions, emperors names were not part of the legend until Tang, but the very first coin with a reign title in the legend was the Han Xing of the Jin dynasty, (Peng Xinwei p.188).

From the Tang to the Yuan emperors adopted new reign titles to mark a new beginning within the reign or after important events. The reign title could be abbreviated to two characters, which were used for the first two characters in the coin legends. From the Tang onwards, the two final characters were Tong Bao (circulating currency), Zhong Bao (heavy currency) or Yuan Bao (primary currency), so from the Tang onwards, there were always four characters on the obverse. 
Emperor Ren Zong of the Northern Song had nine reign titles, coins were cast in 8 of them. His 4th reign title Bao Yuan was changed to Huang Song, because the character bao in his name was the same as the one used on coins, and two baos on a coin would be unsuitable (David Ren p. 53).

In the Ming and the Qing the emperors only adopted one reign title for the whole reign, and the emperors of these two dynasties are mostly known by their reign title instead of their temple name. Emperor Wan Li was an example. His temple name was Ming Shen Zong but his reign title was Wan Li Huangdi. Wan Li was the name used on the coins (Wilkinson p. 109-110). In the Ming and the Qing, coins were no longer called Yuan Bao, since this became the name for silver ingots (Wilkinson p. 249).

The reverse.

The reverse of most coins before the Qing was blank. During the Han, there were Wu Zhus with four rays on the back, and Wang Mang cast a Huo Quan coin with a crescent and a dot. The first coin with characters on the reverse was the Zhi Bai Wu Zhu during the Three Kingdoms period (Ding no. 516-519), but the use of characters on the reverse was not common before the Tang. Symbols on the reverses could be dots, crescents, lines and clouds. Characters on the reverse could be the name of the kingdom, the mint, the year (Peng p. 464), the denomination, the money unit, the government bureau who had issued it, or the character could even be part of a sentence. This was the case with emperor Yong Li of the Ming dynasty, who issued coins with a declaration that was ten characters long, featuring one character on each of the ten coins (Schjöth p. 52 no. 1300-1310). 

Non-Chinese scripts.
China was in some periods ruled by non-Chinese people. This, however, did not generally change the appearance of the coins. They were still round with a square hole, but sometimes the legends were written in a non-Chinese script. One coin of the Western Xia dynasty had the legend written in Tangut script, and many coins of the Yuan dynasty had legends written in Mongol script. The rulers of the Qing were not Han-Chinese but of the Manzu people from the North. In general they adopted the Chinese system of administration and kept the Chinese writing on the obverse of coins, but the words on the reverse were now written in Manzu script. There is a guide about how to read the Qing Manzu legends in Fisher's Ding.

Charms and coins from Vietnam, Japan and Korea.
If you look through a hoard of Chinese bronze coins, you will often find coins that looks like Chinese coins, but after browsing some catalogues you still can't find it. Then there are two possibilities: it can be a coin from one of China's neighboring countries or a charm. 

The legends of Vietnamese, Korean and Japanese coins were written in Chinese characters, and the general design of these coins was imitated from the Chinese coins. The style of the Vietnamese coins was imitated from Chinese coins, but they were generally cast more poorly. The calligraphy of the Korean and Japanese coins was quite different from the Chinese coins.

Charms are good luck coins. They often look like coins, but most often they have some ornamentations on them. Some Chinese charms were official issues, and some were private issues. For more information about charms please visit www.charm.ru where you can also sign up for a discussion group on Chinese coins and charms.