Serif Fonts- for examples Times are usually more traditional and more formal, they often have small extra detail to them. On the other hand there is Sans Serif fonts- such as Arial these fonts tend to be informal and very modern they are known as more 'friendly' fonts.
The poster that is used for the film 'Rocky' uses the font Franklin Gothic Heavy. This font style is large, bold ,very 'in your face' and rather masculine maybe this suggests the type of film. As the film is about a boxer possibly they were trying to portray the character through the title and the style of the title.