These media are used to suppress commensal bacteria while allowing the pathogen to remain viable and grow.
[Source: Ananthnarayan and Paniker's textbook of Microbiology.]
There is also an example of tetrathionate broth as enrichment media given in the book where tetrathionate inhibits coliforms while allowing typhoid-paratyphoid bacilli to grow.
For selective media it writes:
As in the above case (of enrichment media), if inhibiting substance is added to a solid medium, it enables a greater number of the required bacterium to form colonies than the other bacteria.
For this selective media an example of Deoxycholate Citrate Agar (DCA) for fecal samples is given.
What's the difference between enrichment media and selective media? Some answers give quite contradictory statements to the statement of the book like stating that "...it is designed to stimulate growth of many different types of bacterial species" for enrichment media.
I am also not able to explain myself as to how selective medium enables a greater number of the required bacterium to form colonies than other bacteria since inhibitory substance is also present in enrichment media?