The four tones of Mandarin are fundamental aspects of the language for everyone learning how to speak Chinese. Mandarin Chinese, 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. Mastering the tone of each character is difficult for many foreigners learning Chinese, 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: Yinping Yangping Shang Qu
Tone contour: 55 35 21 (4) 51
Tone number: 1 2 3 4

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

Second tone
Second tone, or rising tone (陽平/阳平 yángpíng, literally means yang-level), or linguistically, high-rising: is a sound that rises from mid-level tone to high (e.g., What?!)

Third Tone
Third tone, low tone, or low-falling-raising(上聲/上声 shǎngshēng or shàngshēng, literally means "up tone"): it 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
Fourth tone, falling tone (去聲/去声 qùshēng, literally means "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
The neutral tone is also called the fifth tone or zero tone (in Chinese: 輕聲/轻声 qīngshēng, literally means "light tone"). The neutral tone is sometimes thought of incorrectly as a lack of tone. The neutral tone is particularly difficult for non-natives trying to speak Chinese 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 fully 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, 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.
Examples:
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).
When a 3rd tone occurs before a 1st, 2cd or 4th tone, its shape changes according to the context and 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