If you are referring to another blog wearing a different theme then please provide the URL starting with http://
Fixed means the width of the theme does not change according to screen resolution (or the width of browser) your visitors use.
Fluid means the width of the theme changes according to screen resolution (or the width of browser) your visitors use.
Responsive width means the the layout adapts depending on the size of the device being used to view your site. When responsive width themes are viewed on mobiles the sidebars appear below the posts in order to provide as much space as possible for reading.