It is possible to use the same field on more than one entity - for example, an image field can be used on both users and a node type. You do this by using the row add existing field in the list of fields.
When reusing fields, only parts of the configuration, such as help text, may be changed for each node type or entity. Others must be the same for all entities where the field is used, such as number of values to store.
It is recommended to create new fields, rather than reusing existing ones, unless you have a clear reason to do so. The main reasons for this are: you get increased flexibility for field configuration, and shared fields can make it more difficult to export selected parts of the configuration on your site. (See the section on exporting configuration in Appendix 1 for more information about why configuration export is useful.)
TIP: From a performance point of view, you don't want to have a gazillion fields on your Drupal site. If a content type (or other entity) has a lot of fields, saving the content will result in a lot of data base writes that might take time. Also, the field configuration itself might take a lot of memory to load from the cache (and that is done pretty often).