Before writing below queries first get into your cmd or PowerShell
TYPE:
mongo //To get into MongoDB shell
use <Your_dbName> //For Creating or making use of existing db
To List All Collection Names use any one from below options :-
show collections //output every collection
OR
show tables
OR
db.getCollectionNames() //shows all collections as a list
To show all collections content or data use below listed code which had been posted by Bruno_Ferreira.
var collections = db.getCollectionNames();
for(var i = 0; i< collections.length; i++) {
print('Collection: ' + collections[i]); // print the name of each collection
db.getCollection(collections[i]).find().forEach(printjson); //and then print the json of each of its elements
}