1. In what format can I write a cover letter?
Read the job posting carefully and see if they have any requirements, number one priority is to follow all instructions exactly.
In general you want to start by stating the position you're applying for. After that you can use any format you'd like if you believe it will be professional and beneficial. There is no set format and I've gotten interviews from single paragraph cover letters, and from lengthy ones consisting of 5 or 6 paragraphs. It is about the structure, content, and reason. Just as any writing - say what needs to be said and remove the rest.
2. Can I include technical terms in a cover letter?
Absolutely. I would be be careful not to repeat your résumé though. If you state in your résumé, "skill in technical term" then there is no reason to repeat yourself in the cover letter.
3. Are cover letters sent through email different from written letters?
In general, not really. Just make sure that any minor changes needed are made. For example in an email you might say somewhere "My résumé is attached in both .pdf and .doc for your convenience." Which wouldn't make sense if mailing it in.
MOST IMPORTANT: Follow any instructions given. If the company specifies to state something, state it. If they ask for a format, use it. If they want a particular filetype, do it.