There are many url functions in wordpress which will tell you what is the address of different directories in wordpress.
Let me discuss each of these:-
plugins_url() gives you address of plugins directory. For example http://www.yoursite.com/wp-content/plugins
includes_url() gives you address of wp-includes directory. For example http://www.yoursite.com/wp-includes
content_url() gives you address of wp-content directory. For example http://www.yoursite.com/wp-content
admin_url() gives you address of wp-admin directory. For example http://www.yoursite.com/wp-admin
site_url() gives you address of your site. For example http://www.yoursite.com
home_url() gives you address of your site. For example http://www.yoursite.com
There is slight difference between site_url() and home_url(). Suppose you have saved your core files in wp directory and you want that your website address should be http://www.yoursite.com not http://www.yoursite.com/wp then the home_url() value will be http://www.yoursite.com and it is saved in wp-options table.