Question is very nice.
Lets start from basics
The term employment is basically a contractual relationship between employer & employee, where there is definite & tangible/Intangible consideration for both the parties .
Now in a contractual relationship there are two important elements
The above forms premise for the question raised by you.Pl find point wise reply to your querry
1. Letter of Intent - Law says mere intention to do something, is not binding on the parties involved .Implying that , the agreement is not enforceable in the court of law , in the event of eventuality , say if the candiate does not join or if the employer on or before the date of joining dissolves the said position.Therefore, letter of Intent is basically a document illustrating the willingness of the employer to hire the candidate but may or may not turn into successful hiring .This is generally practiced by the firms who are having water tight and fierce competitors in the market.Generally, it does not carry any compensation/Emolument data .
2. Offer - Again going by the contract Act, In offer there is definite intention to enter into legally binding relationship.offer involves expression of interest by one party to the other , to do or not to do something (Excluding acts of impossible nature or acts which are void ab intio ).In an offer letter the , term of offer are un-ambigous and definite , with intention to create a legal relationship.
3. Appointment Letter - This is basically , a form of contract between the parties involved viz. employer & employee .as such it involves offer + acceptance +enforcibility by the law .(Hence , it qualifies to be a contract )
Avinash Singh 14th January 2010 From India, Rohtak