What is the difference between extend and include in Ruby?

In Ruby, both `extend` and `include` are used to mix in modules into classes, but they are used in different contexts and have different effects.

1. **`include`**: This is used to mix a module's methods as instance methods in a class. When you `include` a module in a class, the methods of that module become available to instances of the class, but not to the class itself.

```ruby
module MyModule
def my_method
puts "This is a method from MyModule"
end
end

class MyClass
include MyModule
end

obj = MyClass.new
obj.my_method # This works
```

2. **`extend`**: This is used to mix a module's methods as class methods in a class. When you `extend` a module in a class, the methods of that module become available at the class level, but not to instances of the class.

```ruby
module MyModule
def my_method
puts "This is a method from MyModule"
end
end

class MyClass
extend MyModule
end

MyClass.my_method # This works
obj = MyClass.new
# obj.my_method # This would raise a NoMethodError since my_method is not available to instances
```

So, in summary:
- `include` is for mixing in instance methods.
- `extend` is for mixing in class methods.

In many cases, you might see `extend self` used within a module, which effectively makes all its methods both class methods (accessible at the class level) and instance methods (accessible at the instance level).

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