Sangam literally means 'assembly 'and at this time, a remarkable amount of poetry was written and collected in Tamil Nadu that is collectively called Sangam literature.
The main point is that,the poetry were written on ordinary people and their lives.

The Bhakti literature began to be composed in the sixth century CE in south India by the Tamil poet-saints.

The Bhakti movement was considered to be most highly recognized and developed in the medieval period.
This movement was put on by those people who were at a time not considered to be suitable for coming into temple ,and were not allowed by priests. Then,These people began a new significance of praying to God,or representing their devotion,without any priests who only thought that they're the only one who could make God listen to the prayers of the people. The Bhakti literature reflects a new form of devotion to God or,a personal bond between the devotee and the God.



So,this is the difference between the two literature...I hope that you got it ...
I would give a detail answer on the basis of these two...in some other question.
Thanks.