What are Salesforce formula fields?

i have text string divides with slashes ,

string contains in one text field by using formula field i need to show from last two slashes text ..

EX :Default Category/Home Video,Default Category/Home Video/Blu-ray/DVD,Default Category/Home Video/Limited Editions

Here :o/p Home video/Limited Editions

Here text varies from record to record   …

i written like :LEFT(RIGHT(Category__c,LEN(Category__c) – FIND(“/”, Category__c)),
FIND(“/”,RIGHT(Category__c,LEN(Category__c) – FIND(“/”, Category__c)-1)))