Div into the content of an Entry
Assuming you are using the Rich Text field, you'll want to look at the Redactor setting, convertDivs.


  convertDivs (Default setting is true)
  
  With this option turned on, Redactor will automatically replace divs to paragraphs.


Go in to your field settings:
Craft Admin &rsaquo; Settings &rsaquo; Fields &rsaquo; Rich Text field name

Look at the config settings to determine which config file you are using. Then, go in to your craft/config/redactor/ConfigFile.json file and edit your redactor settings to exclude the convertDivs setting (assuming nothing else in your config file):

{
    convertDivs: false
}