String and &str both represent text, yet are distinct types. Interacting with values from both types can be an annoying exercise at first as different methods are required to perform similar actions. Prepare yourself for irritating type errors as your intuition develops. Until that intuition develops, however, you will usually have fewer issues if you convert your data to the String type.
A String is (probably) closest to what you know as a string type from other languages. It supports familiar operations such as concatenation (joining two strings together), appending new text onto an existing string, and trimming whitespace.
str is a high-performance, relatively feature-poor type. Once created, str values cannot expand or shrink. In this sense, these are similar to interacting with a raw memory array. Unlike a raw memory array, though, str values are guaranteed to be valid UTF-8 characters.
str is usually seen in this form: &str. A &str (pronounced string slice) is a small type that contains a reference to str data and a length. Attempting to assign a variable to type str will fail. The Rust compiler wants to create fixed-sized variables within a function’s stack frame. As str values can be of arbitrary length, these can only be stored as local variables by reference.
For those readers that have prior experience with systems programming, String uses dynamic memory allocation to store the text that it represents. Creating &str values avoids a memory allocation.
String is an owned type. Ownership has a particular meaning within Rust. An owner is able to make any changes to the data and is responsible for deleting values that it owns when it leaves scope (this is fully explained in chapter 3). A &str is a borrowed type. In practical terms, this means that &str can be thought of as read-only data, whereas String is read-write.
String literals (e.g., "Rust in Action") have the type &str. The full type signature including the lifetime parameter is &'static str. The 'static lifetime is somewhat special. It too owes its name to implementation details. Executable programs can contain a section of memory that is hard-coded with values. That section is known as static memory because it is read-only during execution.