Typically you use widgets or custom post types but in reality these solutions can be unfriendly to clients. WordPress still revolves around the idea of a post based loop so doing traditional “content blocks” that you might find in other frameworks can be difficult. Fortunately there are plugins that are pretty good in this area.
I recommend checking out the following two:
http://wordpress.org/extend/plugins/multiple-content-blocks/
http://wordpress.org/extend/plugins/wp-blocks/