Content-Disposition:What are the differences between “inline” and “attachment”?

What are the differences between

Response.AddHeader("Content-Disposition", "attachment;filename=somefile.ext")

and

Response.AddHeader("Content-Disposition", "inline;filename=somefile.ext")

I don’t know the differences , because when I use one or another I always get a window prompt asking me to download the file for both of them. I read the specs, but it is clueless.