Conventions are ways of doing things, having a navigation bar on the left side of a website is a common convention, for a newspaper a common convention would be having the logo at the top and the slogan just below it. Conventions are useful as the audience become familiar with layouts and will therefore find it easier to navigate around websites etc.
Conventions do not have to be used all the time, sometimes websites can break conventions to appeal to different audiences and stand out from other websites or newspapers. Below is an example of a school website that breaks conventions but that doesnt make it look good.

The usual conventions in web design are the navigation bar on the left with the logo at the top of the website, sometimes pictures are used in the background or to the right of the logo showing children at school.
The website above has the logo at the top with a navigation bar on the left and a picture in the middle.
This website breaks conventions as it has no clear navigation bar and doesnt have much information, but this website is very eye-catching and shows different objects with a book in the middle which had the links that will take you around the website.






