There are five different characters, all mean Middle or Center. Why? Because there were no unified characters before the First Emperor after 221 B.C. So there were many characters for one characters. The above five characters are only part of the characters for Middle or Center.
From the far left to the middle one, all have flags, on the top and on the bottom. The second one from the left has one dot on the flagpole, and two on each side. They are indicative of the middle place between the flags.
The middle one uses a circle to indicate the Middle or Center. The far left is the space in the middle.
The far right has only the flagpole and a square, the second one from right is something between.
Let see the big seal script for Middle or Center
The small seal script for Middle or Center is
There is only one character because small seal script was created by Mr. Si Li under the order of First Emperor to unify the characters. Mr. Li just chose the one with flagpole and the half circle half square one, which is simple and easy to write or draw.
The clerical script for the character is
which is similar to the small seal script, the middle is rectangle now.
The current standard print script is
It is almost same as the clerical script.
Its pinyin is Zhong1.
I need to point out that Chinese call China as 中国. 中 means Middle or Center, 国 or 國 (traditional Chinese) means Country or Nation. So 中国 or 中國 means "Middle Country", or "the Country in the center of the world". Ancient Chinese believed that they were in the center of the world.
Western named China because it was the place where made China, which is not the same meaning as 中国 in Chinese.