Tones

In mandarin (the standard Chinese in China) there are four different tones that seperate words that would otherwise sound the same. The tone of each word when pin yin (Chinese words written using English letters) is used on this website is indicated by a number after the letters. Below is a description of the four tones:

First tone (第一声 di4 yi1 sheng1) - The word is prunounced in a flat manner, i.e. the pitch is the same throughout the prunounciation of that word.
Second tone (第二声 di4 er4 sheng1) - The word is prunounced with a rising pitch.
Third tone (第三声 di4 san1 sheng1) - The word is read with a fast descending pitch followed by a rising pitch.
Fourth tone (第四声 di4 si4 sheng1) - The word is prunounced with a fast descending pitch.

The tones become quite important as your level of Chinese increases, however, when your Chinese is still at a beginners level most Chinese speakers can guess what you're saying even if you're not getting the tones right because of the context.