Well.Hong Kong,Macao and Taiwan use tradtional characters.In China mainland they use Simplified.

The Chinese writing script has over 2500 years of history. It is the same writing script used by Chinese dialects such as Mandarin, Cantonese, etc. "Simplified" Chinese script refers to a new standard of simplified chinese characters which was instituted by the People's Republic of China, which gained control of the Chinese mainland in 1949, but does not rule all Chinese people. By contrast, the Republic of China (the government in Taiwan) and other Chinese-speaking communities do not use the new simplified writing script. Their chinese script is the same that has been used for millennia (although there is some minor changes over the years) and is now referred to as "Traditional" Simplified chinese is a relatively new innovation (late 1950's), it was done to help improve literacy - in fact Chairman Mao (leader of People's Republic of China) wanted to completely do away with Chinese script and use roman alphabet instead, but later the PRC (communist government in mainland China) abandoned these plans, as I recall. It's also worth noting that some simplification existed prior to the introduction of a new standard, because Chinese cursive writing uses simplified characters (how else could you write fast?) In my experience, most people who use simplified Chinese understand traditional Chinese without problems.

It's OK to use 車 instead of 车, just make sure of its concordance of the character type in your writing.

Yes, it's ok if you use traditional, but avoid mixing them together. You'll be understood. Chinese can read both simply and traditional characters. Most ppl from HK can only read tradictional tho. So, yeah, you'll be understood.=)

Ian's rundown is pretty good. I'm learning simplified characters now because I'm going to work in mainland China, but Hong Kong and Taiwan still use traditional characters- same thing with a lot of Chinatowns in various countries. Ideally, I'd like to learn both systems; friends who know both have told me that there are a few rules for simplification that apply to most of the characters and that once you know those, it's not horribly difficult to switch between the two. I learned Japanese prior to studying Mandarin, and what surprised me was that some of the Japanese characters are identical to the simplified ones, while others are traditional. A Taiwanese friend said it's because some of the simplifications instituted in the PRC weren't totally new, just throwbacks to older simplifications that the Japanese took with them when they borrowed the Chinese writing system. I found that interesting.