Your WordPress site is built on two different content types: Posts and Pages.  They are very similar in functionality so I will list the differences below.

Posts

• Think of these as news articles which are to be updated on a regular basis
• Listed in reverse chronological order 
• You can sticky posts which will place them at the top of the list
• You add categories and tags to posts to help with sorting and searching

Pages

• Static and not listed by date
• Basic content type for your website that is not updated often