chemical hair straightener rearranges the structure of the hair from curly to straight. The chemicals usually used are sodium hydroxide, guanidine hydroxide and ammonium thioglycolate. Using chemical straightener could be dangerous, so we advise you to let a professional do it (remember, you may cause a permanent damage to your hair).
The hair straightener makes a temporary change for the formation of your hair and not a permanent one like the chemical straighteners. If you are not sure you want to have a flat hair for the rest of your life use a hair straightener. However, the hairstraightener requires a daily use of at least 10 minutes. It is not expensive and it can be used by anyone. The risk of causing severe damage to the hair is less significant.