Entry fields are dealt with via “Field Groups”.  You create a Field Group and then create individual entry fields within it.  You can create as many Field Groups as you want and you can have as many individual entry fields within each Group as you want.

Entry fields in two different Groups can have the same label (say, “Main Content") but they may not have the same short name for the field.  Every entry field you create, regardless of Field Group must have a unique short name (say, “news_main_content").

When you create a weblog you assign a Field Group to it.  The same Group can be used with more than one weblog or you can create a separate Group for each weblog. A weblog may only have one custom field group assigned to it.

See Custom Weblog Fields in the documentation.