For those who might not read the question fully: this not about simplified vs traditional characters. While the traditional and simplified version of a character can be considered the same, they at least have different code points, and I am well aware of the existence of traditional forms, so I won't get confused by them.
First, some explanation:
I noticed that my browser was displaying some Chinese characters in a way that's unfamiliar to me (I'm a beginner learner). To cut a long story short: the reason is that the same character (with the same Unicode code point) may look different in a Japanese and a Chinese font. English-language operating systems will typically choose a Japanese substitute font in almost any program when the main font doesn't include CJK glyphs.
Here's an example of several characters that will typically appear markedly differently in a Japanese or Simplified Chinese font (Meiryo and YaHei on Windows):
Note that sometimes even the stroke count differs, e.g. in 直! Traditional Chinese fonts can be slightly different from Simplified Chinese ones too. This is quite frustrating for me as a beginner because as you know sometimes the visual difference between truly different characters (like 八人入) is even smaller than the visual difference between these stylistic variants of identical characters on the image above.
Now, the questions:
Are all of these different styles of the same characters used in China as well, or are they specific to Japan? Is it important for me to learn about them (i.e. learn both shapes of 令 or 直)? Note that Traditional Chinese fonts (like MingLiu) will often have different versions from Simplified Chinese ones for the same Unicode code point (though not as extremely different as the Japanese one).
What other common characters are there which may look significantly different in different-language fonts?
Should I just think of these differences as similar to the common stylistic differences in the small Roman letter 'a' and 'g', like in the image below?
Of course I'm aware that there are several calligraphy styles, but note that I'm a beginner (i.e. I definitely won't try to read 草书) and this is about common print styles I came across on a computer.
Interesting reading: http://en.wikipedia.org/wiki/Unihan#Examples_of_language_dependent_characters
(Note: Doesn't work in WebKit. It will work in Mozilla browsers or IE on Windows. It's unlikely to work on Linux due to bad font support. I don't know about Mac.)