Last time I discussed learning Mandarin I mentioned a few things about characters. Today I'm talking about tones.
In Mandarin there are 4 tones (or 5 if you count the 'neutral' tone). The tones are numbered 1 through 4 and represent the pitch of the word. The first tone is high pitch, the second tone is rising, the third tone is falling-rising, and the fourth tone is falling.
Tones are very important because there are lots of words that are indistinguishable except for the tone. The classic example are the words mā (媽) and mǎ (馬). The first word means mother and the second means horse. The first word is pronounced with the first tone while the second is pronounced with a 3rd tone. This musicality is the only way to distinguish between the two words when spoken in isolation. In a complete sentence, you can sometimes guess the word based on the context, but it can be unclear.
Consider the following example:
English: Please ask me.
Chinese: Qǐng wèn wǒ.
If you mispronounce the middle word, using the 3rd tone instead of the 4th tone, wèn (問) becomes wěn (吻), which changes the sentence to "Please kiss me". While this could be an extremely useful phrase, using it in the wrong situation can lead to problems.