From the Japan SAQ (Seldom Asked Questions):
Q. What is the difference between hara-kiri and seppuku?
A. Some people will tell you that seppuku is only used to describe the act of disemboweling oneself when it is committed by a samurai, and that hara-kiri is the term applied to disembowelment by commoners, but this is not true. They are two different words for exactly the same thing. The word hara-kiri is less formal than the word seppuku, but there is no other difference and the words are written with the same two kanji. Seppuku is a contraction of the kanji setsu (cut) and fuku (abdomen) which are pronounced differently when they are reversed in the word hara-kiri.
Read the whole SAQ; it’s fascinating.