Mandarin, like most Chinese dialects, is a tonal language. This means that tones, just like consonants and vowels, are used to distinguish words from each other. Many foreigners have difficulties mastering the tones of each character, but correct tonal pronunciation is essential for intelligibility because of the vast number of words in the language that only differ by tone (i.e. are minimal pairs with respect to tone). The following are the four tones of Standard Mandarin:

Tone name Yin Ping Yang Ping Shang Qu
Tone contour 55 35 21 (4) 51
Tone number 1 2 3 4

First tone, or high-level tone (陰平/阴平 yīnpíng, literal meaning: yin-level): a steady high sound, as if it were being sung instead of spoken.

Second tone, or rising tone (陽平/阳平 yángpíng, literal meaning: yang-level), or linguistically, high-rising: is a sound that rises from mid-level tone to high (e.g., What?!)
Third tone (low tone, or low-falling-raising, 上聲/上声 shǎngshēng or shàngshēng, literal meaning: "up tone"): has a mid-low to low descent; if at the end of a sentence or before a pause, it is then followed by a rising pitch.
Fourth tone, falling tone (去聲/去声 qùshēng, literal meaning: "away tone"), or high-falling: features a sharp downward accent ("dipping") from high to low, and is a shorter tone, similar to curt commands. (e.g., Stop!)

Neutral tone

Also called Fifth tone or zeroth tone (in Chinese: 輕聲/轻声 qīng shēng, literal meaning: "light tone"), neutral tone is sometimes thought of incorrectly as a lack of tone. The neutral tone is particularly difficult for non-native speakers to master correctly because of its uncharacteristically large number of allotone contours: the level of its pitch depends almost entirely on the tone carried by the syllable preceding it. The situation is further complicated by the amount of dialectal variation associated with it; in some regions, notably Taiwan, neutral tone is relatively uncommon. Despite many examples of minimal pairs (for example, 要是 and 钥匙, yàoshì if and yàoshi key, respectively) it is sometimes described as something other than a full fledged tone for technical reasons: namely because some linguists have historically felt that the tonality of a syllable carrying the neutral tone results from a "spreading out" of the tone on the syllable before it. This idea is appealing intuitively because without it, the neutral tone requires relatively complex tone sandhi rules to be made sense of; indeed, it would have to have 4 separate allotones, one for each of the four tones that could precede it. Despite this, however, it has been shown that the "spreading" theory inadequately characterizes the neutral tone, especially in sequences where more than one neutrally toned syllable are found adjacent.

The following are from Beijing dialect. Other dialects may be slightly different.

Tone of first syllable Pitch of neutral tone Example Pinyin English meaning

1 2 玻璃 bōli glass

2 3 伯伯 bóbo uncle

3 4 喇叭 lăba horn

4 1 兔子 tùzi rabbit

Most romanizations represent the tones as diacritics on the vowels (e.g., Pinyin, MPS II and Tongyong Pinyin). Zhuyin uses diacritics as well. Others, like Wade-Giles, use superscript numbers at the end of each syllable. The tone marks and numbers are rarely used outside of textbooks. Gwoyeu Romatzyh is a rare example where tones are not represented as special symbols, but using normal letters of the alphabet (although in a very complex fashion).

The shape of the 3rd tone when before 1st, 2nd and 4th tonesPronunciation also varies with context according to the rules of tone sandhi. The most prominent phenomenon of this kind is when there are two third tones in immediate sequence, in which case the first of them changes to a rising tone. This tone contour is sometimes described incorrectly as being equivalent to second tone; while the two are very similar, many native speakers can distinguish them (compare 起码 and 骑马, pinyin qĭ mă and qí mă respectively). In the literature, this contour is often called two-thirds tone or half-third tone. If there are three third tones in series, the tone sandhi rules become more complex, and depend on word boundaries, stress, and dialectal variations.

From: wikipedia - Chinese Four Tones