A Mast Head is the title of a magazine. It's always at the top because it is the first place you look, and on a magazine rack it is all that would be showing so the public would be able to see it. The font is also choosen in accordance with the genre of the magazine. A girly magazine is going to ave swirly firty style writing wheras a magazine for computers is going to be quite basic and have a standard font.The colour of the magazines writing also allows the reader to determine what sex the magazine is aimed at. Pink is more likely to be for girls and blue for boys etc..
A tagline goes near the masthead it will normally show what the magazine is about.
The central image is one of the most important parts of the front cover it catches readers eyes befor text. The central image will be relevant to the magazine's purpose and audience, and will often be related to the feature article.