When object's are linked by a lookup or master-detail relationship, the parent records field's can be referenced from the child record or 'base object' in a query. This is also known as upwards traversal.
SELECT FirstName, Account.Name, Account.Category__c FROM Contact
It's possible to traverse five records upwards.
SELECT Account.Owner.Profile.CreatedBy.Name FROM Contact
When the base object is a custom lookup field, the __c in field's name Primary_Influencer__c, for example, will be changed to __r.
SELECT Primary_Influencer__r.Nickname__c FROM Contact
SOQL Query to get child Records
SELECT Id, Name, (SELECT Id, FirstName, LastName FROM Contacts) FROM Account