A design pattern is a recurring solution to a common problem. A solution for a problem that has been found to work well over and over. Such a solution over time becomes a convention.
Conventions are not limited to the online world, but is to be found everywhere in real world as well. As we grow up, we learn the convention of pushing a door-handle down to open its door and driving on the right side of the road (depending on what country you live in) to avoid nasty collisions. We learn to cross a street when the light is green and hold when its red. We learn how to read a newspaper by learning the conventions of page layouts and formatting to easier scan its information. In this way we can find the stories we are interested in quickly and efficiently.
Conventions all start off as an idea good enough to be imitated by others. Similarly, design conventions on the web stem from ideas on one website good enough to be imitated by another. As more websites copy an idea, it is exposed to the public as more and more see it. When it has reached a critical mass, it needs no explanation. This is when it has become a convention. The idea that solved a problem becomes a convention if it is common enough. If it also has a good amount of quality, it is a design pattern worth talking about.
Conventional design patterns provide a reassuring sense of familiarity to the user. When one is spotted the user immediately knows how it works and what it does. No unnecessary time needs to be spent to figure out how things work. The user does not need to think.
Take advantage of design patterns and the conventions embedded in them. Using design patterns may not win you the pulitzer prize for new and innovative thinking, but they will help you reduce friction and thus provide better usability. Innovate when you know you have the better idea, but use design patterns and conventions in all other cases.
It’s all about the execution.