The core modules are packaged with the content module, so there is no additional download needed to make them available, but they do need to be checked on the modules page to install them.
The text module allows you to create fields that are stored in the database as text. If you give a text field more than 1 row it will be stored in the database a a text field and presented to the user as a textarea (like the 'body' field in current content types). If you give a text field 1 row, it will be stored in the database as a varchar field and presented to the user as a textfield.
The number module provides a way to create two kinds of fields, an integer field and a decimal field.
The nodereference module allows you to create links between nodes by selecting related nodes from either a drop-down list or an autocomplete field.
The userreference module allows you to create links between nodes and users by selecting related users from either a drop-down list or an autocomplete field.
The optionwidgets module can be used with the text or number modules to create drop down selectors, radios, or checkboxes from a list of allowed values.
The fieldgroup module allows you to group fields together in fieldsets to help organize them.