In general every Item is associated with a Template, and the Fields defined on the Template define the fields which appear on the Item.

In object-oriented programming terms a Sitecore Item is most like an object, an instance of a class (Template).  In relational database technology an Item is most like a row in a table (Template).  Users work with Items, which are also generally aggregated to render content on the Published Site.  An Item consists of a series of Fields which are associated with data types such as HTML, date, multilist, etc.  Fields are defined in the Template associated with the Item, which by default is the Template that was used to create the Item.  If an Item associated with one Template needs to be associated with an alternate Template, data in the Item will be maintained after the change if field names in the new Template match the field names in the original Template.  For instance, if both Templates have a field named Title, that field value will be maintained, but if the original template has a Headline field and the new Template does not, that field value will be lost.

Only Administrators can create Items from Templates; most users can only create Items using Masters associated with existing Items by an Administrator.  Another important distinction between Templates and Masters is that Master configuration is only applied when an Item is created – changes to a Master do not affect Items created before that change.  This is generally most important for security and layout settings – updating these settings on a Master will not update Items previously created with that Master.  This is different than with Templates, where adding a Field at any time immediately adds that Field to all Masters and Items associated with that Template.

Note that Items may sometimes be referred to as nodes or items, but should not be called documents to avoid confusion with PDFs and other file types.