They are the same thing. Spell it "template." "Templet" is a very rare spelling. I am surprised to find that it really is in the American Heritage Dictionary as an "alternative to 'template.'" However, I'm a U.S. speaker and I have never, ever, EVER seen it spelled that way until I read this question. When I do a Google search on "templet" it asks "Do you mean template?" and reports 3,430,000 results for "templet." It reports 850,000,000 results for "template."

I agree with Bill and Jerry about it being the same meaning. They are pronounced the same. When I was young, I did see the "templet" word occasionally in textbooks. In the last 30 years, I have not seen the word spelled that way.

They have the same meaning but I've never seen it in American English. Ever. As Bill said, use template.

template is the more usual word. templet is not used in British English and is only occasionally used in American English, but the two words mean the same thing. So just use template