In Kerala, what is the difference between a Syrian Catholic and a Roman Catholic? And can they marry each other?
The short answer is that any two Catholics, of any rite, have the right to marry one another (provided the usual conditions are met; i.e., that the couple consists of one man and one woman, and that both are free to marry).
The groups mentioned by the O.P. are simply autonomous churches that are both part of the Catholic Church, hence in full communion with one another.
Syrian Catholics vs. Latin Catholics
The Catholic Church, in addition to the Latin or Western Church that is the best well known, also has 23 autonomous Eastern churches in full communion with the universal Church. Among these are the two largest Eastern churches in India, the Syro-Malabar Church and the Syro-Malankar Church. Since both churches use a liturgy that is derived from the Syriac liturgy, they are sometimes called “Syrian Catholics.”
The O.P. used the term “Roman Catholics,” but that term is now seldom used in official Church documents. The more precise term for the largest group of Catholics is “Latin” or “Western” Catholics, to distinguish them from those who follow Eastern liturgical traditions.
However, in terms of their belonging to the universal Church, and being in communion with the Bishop of Rome (that is, the Pope), there is no difference between Western Catholics and Catholics from the Syro-Malabar or Syro-Malankar Church.
Marriage between Catholics of Different Catholic Churches
The Churches in full communion with the Catholic Church enjoy complete freedom to celebrate all the Sacraments in common, including matrimony.
That members of different autonomous churches may marry is implied by various canons of the Code of Canon Law, including the following:
Can. 112 §1. After the reception of baptism, the following are enrolled in another ritual Church sui iuris:
2/ a spouse who, at the time of or during marriage, has declared that he or she is transferring to the ritual Church sui iuris of the other spouse; when the marriage has ended, however, the person can freely return to the Latin Church;
3/ before the completion of the fourteenth year of age, the children of those mentioned in nn. 1 and 2 as well as, in a mixed marriage, the children of the Catholic party who has legitimately transferred to another ritual Church; on completion of their fourteenth year, however, they can return to the Latin Church.
Can. 1109 Unless the local ordinary and pastor have been excommunicated, interdicted, or suspended from office or declared such through a sentence or decree, by virtue of their office and within the confines of their territory they assist validly at the marriages not only of their subjects but also of those who are not their subjects provided that one of them is of the Latin rite.
Similarly, the Code of Canons for the Eastern Churches says
Canon 33 A wife is at liberty to transfer to the Church of the husband at the celebration of or during the marriage; when the marriage has ended, she can freely return to the original Church sui iuris.
Canon 34 If the parents, or the Catholic spouse in the case of a mixed marriage, transfer to another Church sui iuris, children under fourteen years old by the law itself are enrolled in the same Church; if in a marriage of Catholics only one parent transfers to another Church sui iuris, the children transfer only if both parents consent. Upon completion of the fourteenth year of age, the children can return to the original Church sui iuris.
Canon 829 §1 From the day of taking canonical possession of office and as long as they legitimately hold office, everywhere within the boundaries of their territory, local hierarchs and pastors validly bless the marriage of parties whether they are subjects or non-subjects, provided that at least one of the parties is enrolled in his Church sui iuris.