Within SharePoint it is possible to create two different types of column/field, these are site columns and list columns. Site columns are rather like templates which are created on a site root and can then be added to any lists within that site or child sites as a list column. Alternatively it is possible to create a list column which will only exists on that particular list and is not based on a site column.
List columns which are based on site columns will maintain the site column ID. Columns are handled in a very similar manner to content types and any changes made at the site level can optionally be pushed down to the child columns, but be warned that this could potentially mean losing custom amends to individual list columns.
The quoted paragraph below sums it up nicely:
Updating Site Columns
If you make changes to a site column, you can update all the list columns based on that template. This is done by pushing down the changes you made to the site column. A list column retains the same column ID as the site column on which it is based; when you push down the changes to a site column, SharePoint Foundation identifies all the list columns based on that site column and updates them.
For more detailed information take a look here: