what is the difference between mille and millier de

Deng, 

Link to a previous answer by Céline to similar question. 

They are interchangeable terms, except when there is a complement following, in which case "mille" should not be used. 

For example,  ' ... de milliers de visiteurs' is correct, but not '.... de mille de visiteurs" - a limitation on the use of 'mille'.

See Larousse under Difficulties-Emploi

 https://progress.lawlessfrench.com/questions/view/mille-et-milliers

 https://www.larousse.fr/dictionnaires/francais/mille/51453#difficulte

For those like me who thought Maarten's answer seemed to rule out "mille" altogether, the complement rule in the Larousse article is about usage following "des dizaines de...".