They sound very similar and they both have something to do with pork but they are very different!
You are most likely to see the word トンコツ（tonkotsu） in a ramen shop. トンコツ literally means pig bones, so ramen shops whose broth is made of pork bones has a sign saying “豚骨ラーメン（トンコツラーメン/tonkotsu ramen）”.
On the other hand, トンカツ（tonkatsu） is a crumb fried “pork cutlet.” “カツ（katsu）” apparently comes from the English word “cutlet” but Japanese usage of “カツ（katsu）” is “crumb fried meat”, so you can have ビフカツ（bifu_katsu, crumb fried beef), チキンカツ（chikin_katsu, crumb fried chicken) and メンチカツ（menchi_katsu, crumb fried rissoles).
If inside the crumb fry is not meat, then they are called フライ（hurai）. エビフライ (ebi_hurai, crumb fried prawns), カキフライ (kaki_hurai, crumb fried oysters), 魚のフライ (sakana_no_hurai, crumb fried fish), etc.
As not many places serve both トンコツ and トンカツ at the same time, you will not have a catastrophe even if you mix them up!