There is an ocean of differences between these two avenues of marriage.
Forced Marriage: This is a marriage against a persons’ will. It often occurs with underage girls with older men. But this can also happen for adults as well. It is a marriage which is against the consent of one or both parties. This has happened in some countries where a person can be sold into marriage or it is a marriage of convenience for the family but not of the bride and groom.
Simply put a forced marriage is a union against the will of the bride or groom or both.
Arranged Marriage: This is a different set up and is often with consent that a couple allow their family to arrange for their marriage. This is a very common practice particularly in places like India. It is thought that who better to choose an appropriate bride or groom than family. This is an acceptable practice and it is with consent from the bride and groom that such a choice is made for them. In these cases, they are consenting adults and it is considered a time honoured tradition that the family choose someone suitable for their son or daughter. This is not a forced marriage because if the bride or groom do not like who has been chosen for them they do have the choice to say no so therefore this is not a forced marriage but rather an acceptable arranged marriage.
Sometimes it is nice to understand the difference.