Intersect

Intersect[ list1, list2, ...]

Intersect[ list1, list2, ...] returns the list of all elements that occur in all lists list1, list2, ..... The resulting list contains duplicates only if all the lists contain duplicates.

Example in out
The intersection of {a,b,g} and {a,b,c,d,e,a} Intersect[ {a,b,g}, {a,b,c,d,e,a} ] {a,b}
The intersection of {a,b,g,a} and {a,c,a}. Intersect[ {a,b,g,a}, {a,c,a} ] {a, a}
The intersection of {a,b,g,a}, {a,b,c,d,e,a} and { g,d,f,a}. Intersect[ {a,b,g,a}, {a,b,c,d,e,a}, { g,d,f,a} ] {a}
The intersection of x and y, where x is defined as {a,b,c} and y is defined as {b,c,d} in the script. Intersect[x, y] {b,c}