Character Encoding and the ’ Issue

Question

Even today, one frequently sees character encoding problems with significant frequency. Take for example this recent job post:

(Note: This is an example, not a spam job post... :-)

I have recently seen that exact error on websites, in popular IM programs, and in the background graphics on CNN.

My two-part question:

• What causes this particular, common encoding issue?
• As a developer, what should I do with user input to avoid common encoding issues like this one? If this question requires simplification to provide a meaningful answer, assume content is entered through a web browser.