In easy words, String is datatype stored on heap (just like Vec), 

and you have access to that location.

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&str is a slice type. That means it is just reference to an already 

present String somewhere in the heap.

​

&str doesn't do any allocation at runtime. So, for memory reasons, 

you can use &str over String. But, keep in mind that when using 

&str you might have to deal with explicit lifetimes.