http://en.wikipedia.org/wiki/Kanji
That page answers pretty much everything and isn't exactly hard to search for.
Basically...
When the kanji were imported from China into the Japanese language, the sounds of the single Chinese pronunciations for each kanji were brought over with them. These sounds that were brought over with the kanji are called the 音読み (on'yomi), which means "sound reading".
However, Japanese was a spoken language before it was written down. So, for the pronunciations of these words already in the Japanese language, kanji with similar meanings needed to be chosen which to write these original Japanese words with. So these Japanese words were added as extra readings to kanji. These readings are known as 訓読み (kun'yomi), which basically means "instruction reading" (like new instructions/explanations given to the kanji).
As kanji were imported over from Japan over the course of a few hundred of years, and because the kanji were being imported from different regions of China, there are now often more than one single on'yomi for each kanji. Some on'yomi are mistaken or modified pronunciations, not the same as what should have been imported with the kanji.
A few kanji don't even have an on'yomi. There have also been a few kanji created in Japan that don't exist in China. These, understandably, usually don't have an on'yomi. A single kanji can have many kun'yomi, but there are also many kanji which only have on'yomi and have no kun'yomi.
I definitely recommend trying to read that whole Wikipedia article, or at least searching Google for more information on this topic. This following link should be helpful to bookmark for later on in your studies. :)
http://lingwiki.com/index.php?title=On_vs._Kun_readingsDecember 9, 2014 at 5:57am
*do
onyomi refers to kanji chinese readings and
http://en.wikipedia.org/wiki/Kanji