The first tone is high and level. It is important to keep one's voice even (almost monotone) across the whole syllable when pronouncing the first tone. It is represented by a straight horizontal line above a letter in pinyin.  

The second rises moderately. In English we sometimes associate this rise in pitch with a question. The second tone is represented by a rising diagonal line above a letter in pinyin. 

The third tone falls and then rises again. When pronounced clearly, its tonal "dipping" is very distinctive. It is represented by a curved "dipping" line above a letter in pinyin.

The fourth tone starts out high but drops sharply to the bottom of the tonal range. English-speakers often associate this tone with an angry command. It is represented by a dropping diagonal line above a letter in pinyin.