Instead of defining a type with
class you can do so with
struct Point property x, y def initialize(@x : Int32, @y : Int32) end end
The differences between a struct and a class are:
newon a struct allocates it on the stack instead of the heap
The last point has a reason to it: a struct has a very well defined memory layout. For example, the above
Point struct occupies 8 bytes. If you have an array of points the points are embedded inside the array's buffer:
# The array's buffer will have each 8 bytes dedicated to each Point ary =  of Point
Point is inherited, an array of such type must also account for the fact that other types can be inside it, so the size of each element must grow to accomodate that. That is certainly unexpected. So, non-abstract structs can't be inherited. Abstract structs, on the other hand, will have descendants, so it's expected that an array of them will account the possibility of having multiple types inside it.
A struct can also includes modules and can be generic, just like a class.
A struct is mostly used for performance reasons to avoid lots of small memory allocations when passing small copies might be more efficient.
So how do you choose between a struct and a class? The rule of thumb is that if no instance variable is ever reassigned, i.e. your type is immutable, you could use a struct, otherwise use a class.