It's 2 sets of Characters in the first place and are used under different circumstances, where the same character may have different when written in 2 sets. Multiple simplification standards were applied, some were abandoned and some remain till today. The purpose of the simplification is to decrease the percentage of 文盲(uneducated people, lit. "people that cant read") by decreasing the complexity of the characters.(Some of the linguists suggested latinization, but they didn't succeed).
During the process, some phonetically-similar Characters were simlipied into a single character.
The Chinese Languages used in Taiwan, Hong Kong and Macau regions adopts the traditional Characters, because those regions were not governed by the Chinese Gov in Mainland when the simplification took place only in China Mainland.
To be more precise, the Simplified Chinese you saw when translating often refers to Manderin of China Mainland, and Traditional Chinese often refers to Manderin of Taiwan. You can see that they are assigned the names"zh_CN" and (often) "zh_TW". And some phrases representing the same thing are different between zh_CN and zh_TW.
The most widely spoken Chinese languages are Manderin and Cantonese. If you plan to learn Cantonese, i'd suggest you to learn the traditional Chinese Characters. But if you plan to learn Manderin, it depends on where and with whom you want to use the language. If you are to use Manderin with mainly Taiwanese, i'd suggest you to get to the traditional characters from the beginning. Otherwise(and more likely) you can just start with the simplified ones, they cause less trouble learning characters and you can thus take more time to adopt the other language features. If you have more passion, you can learn both in parellel.
The same text can be written in either set of characters. Note that for some other major dialects(Cantonese or Hakka and so on) have VERY different grammar and vocabulary from Manderin, and some of their written forms adopts the traditional characters and exclusive characters.