Which is the main difference between diffrent types of map projections?

Mercator projection maps show both latitude and longitude lines being parallel from each other, respectively (latitude lines being parallel to each other, etc). In other Words, mercator maps are shown in grid form (looking like paper was wrapped around the globe in cylindric form and copied down)

Conic projection maps show the globe being viewed from "outer space", like a paper was formed into a cone and the tip would be where you're looking down at the world. In other Words, conic projection maps show the world from a bird's eye view.