See the documentation: grep and map. Briefly (not not entirely accurately):

grep expression list runs across a list and returns a list containing the elements of the original list for which the expression is true.

map expression list runs across a list and returns a list containing the value of expression for each of the elements of the original list.

Excellent advice on this is given in the classic Shiny Ball Book, Item 12 "Use foreach, map and grep as appropriate":

• Use foreach to iterate read-only over each element of a list
• Use map to create a list based on the contents of another list
• Use foreach to modify elements of a list
• Use grep to select elements in a list

Have a look at the following URL

The grep operator taking list of values and a "testing expression". It takes one value after another and put it into the $_ then evaluate the testing expression in a scalar context. If expression evaluates to a true value it will return the $_ into the output list. In a list context grep return the list of selected items. In a scalar context grep return the number of selected items.

The map operator has a very similar step to grep operator and shares a lot of the same operational steps. It is also temporarily place the items from the list into $_ one at a time. The map operator evaluates the expression in a list context( not a scalar context like grep). Each evaluation of the expression gives a portion of the many results. the overall result in the list concatenation of all individual results. map return no.of elements that are returned in a list context.