How is an LOR presented when applying for grad school? I am going to apply for grad school starting this December. I have heard that an LOR is pretty essential in the process. I have a few professors I can approach. But my question is: how do you present an LOR? By that I mean, should it be sealed in an envelope, giving the impression that I have not peeped into it, before submitting it for grad school? 

What if my professor mails me an LOR instead of writing it? Do I take a printout and do the above?

Also is there a particular format I should be aware of? And what else should be there on the letter, apart form the content itself, college stamp/seals or anything of the sought?

Are there any other formalities that I should be concerned about?
The rules vary greatly by country and program. You should follow the rules as specified by each individual program. 

For instance, in the US, the applicant should never see a letter of recommendation at any stage of the admissions process. Nowadays, this is circumvented by having the letter-writer submit the reference directly, with the applicant only "supervising" the process to see if a letter has been submitted. In the rare cases where everything is to be submitted together, it is usually required that the letter writer sign across the seal of the envelope, to ensure no tampering has occurred. 

As for the format, it should again follow local customs—for example, in the US, being submitted on official letterhead is sufficient; a mailed letter in Germany would be considered suspect without a stamp mark. (An emailed letter would be an exception to this, but would still be expected to have an electronic or digital signature of some sort.)