As mchj2468 mentioned, there's four parts to Japanese Writing.
However, the 2,136 Kanji to learn are NOT the only ones out there.
In reality, only 1,945 are usually used, and the other 191 are occasionally used.
There's also the Jinmeiyou (人名用; Person Name Use) Kanji which are specifically for names.
[Jinmeiyou include characters which are and are not part of the Jouyou (常用; Common Use) set.]
Roumaji (ローマジ) is only useful when it is used correctly, but doesn't work properly with homonyms (of which there are several).
As you learn how to use Kanji more and simple things less, you'll understand why they are used.