In the Rust programming language, `struct` (struct) is a custom data type that allows you to name and group multiple related values into meaningful combinations. It is similar to classes in other languages but does not include methods (methods can be associated with the struct via `impl` blocks). Structs are primarily used for creating complex data types that can contain fields of different types, which are accessed via field names.

Rust has several types of structs:

1. **Named structs**: contain named fields.
   ```rust
   struct Person {
       name: String,
       age: u8,
   }
   ```

2. **Tuple structs**: essentially named tuples.
   ```rust
   struct Color(u8, u8, u8);
   ```

3. **Unit structs**: contain no fields and are typically used to express certain traits at the type level.
   ```rust
   struct Marker;
   ```

Using structs can enhance code modularity and readability, while also facilitating data management and operations.

What is a struct in Rust?