If you have just had a telephone interview and you want to follow this up with a simple thank you then it is important to make notes on your telephone interview as you go
If you have been offered a new job and you want to write back to your employers when you have received your job offer to say thank you then this is a nice and respectful thing to do.
If you have carefully decided that you do not want to accept a job offer then you need to word your letter carefully and considerately. Remember that you need to have good reasons
If you have sent your CV in and are waiting for a response from a company, please remember that not all companies will get back to you with an acknowledgement from just sending in a CV.
If you have just had a group interview for a position that you have applied for and you want to express your thanks and gratitude to the people involved in that interview