Both the E-R model and the relational model are abstract, logical, representations of a given reality. The two models employ similar design principles and it is therefore possible to convert an E-R design into a relational one (and vice versa).
Formally, the constructs of an E-R diagram are converted into relations. The conversion of an E-R diagram into a relational database schema is actually a translation process based on a few translation rules.
An E-R diagram needs to be restructured (cfr E-R Restructuring), to resolve all the problematic constructs, before this translation process can begin.
In principle, each entity set and each relationship set will result in a corresponding relation. In practice, some relationship sets may be disregarded, in some particular situations, to avoid data redundancy.