struct

A struct type is a value type that is typically used to encapsulate small groups of related variables, such as the coordinates of a rectangle or the characteristics of an item in an inventory.

Classes are reference types, structs are value types.

using static System.Console;

namespace ConsoleApplication1
{
    struct Point
    {
        public int X;
        public int Y;

        public override string ToString()
        {
            return $"X = {X}, Y = {Y}";
        }

        public void Display(string name)
        {
            WriteLine(name + ": " + ToString());
        }
    }

    class Program
    {
        static void Main()
        {
            var point1 = new Point {X = 10, Y = 20};
            // it's not a reference but value type
            var point2 = point1;
            point2.X = 777;
            point2.Y = 888;
            point1.Display(nameof(point1)); // point1: X = 10, Y = 20
            point2.Display(nameof(point2)); // point2: X = 777, Y = 888

            ReadKey();
        }
    }
}

Structs can also contain constructors, constants, fields, methods, properties, indexers, operators, events, and nested types, although if several such members are required, you should consider making your type a class instead.

Some suggestions from MS on when to use struct and when to use class:

CONSIDER

defining a struct instead of a class if instances of the type are small and commonly short-lived or are commonly embedded in other objects.

AVOID

defining a struct unless the type has all of the following characteristics:

• It logically represents a single value, similar to primitive types (int, double, etc.)
• It has an instance size under 16 bytes.
• It is immutable.
• It will not have to be boxed frequently.