In Python 3, a `class` is a code template for creating objects. Objects are instances of a class and have attributes (properties) and methods (functions) that are defined within the class. To define a class in Python 3, you can follow these steps:

1. Use the class keyword followed by the class name (by convention, class names use CamelCase).
1. After the class name, add a colon to indicate the beginning of the class body.
1. Indent the class attributes and methods to define them within the class body.

Here's a simple example of how to create and use a class in Python 3:
```python
# Define a class named 'Person'
class Person:
    # Define the constructor method with the 'self' parameter, which refers to the instance of the class
    def __init__(self, name, age):
        self.name = name
        self.age = age

    # Define a method named 'greet' that prints a greeting message
    def greet(self):
        print(f"Hello, my name is {self.name} and I'm {self.age} years old.")

# Create an object (instance) of the 'Person' class
person1 = Person("Alice", 30)

# Access the object's attributes
print(person1.name)  # Output: Alice
print(person1.age)   # Output: 30

# Call the object's method
person1.greet()  # Output: Hello, my name is Alice and I'm 30 years old.
```

In this example, we define a `Person` class with a constructor method (`__init__`) and a `greet` method. The constructor is called when a new instance of the class is created, initializing the attributes `name` and `age` with the values passed as arguments. We then create an instance of the `Person` class, access its attributes, and call its `greet` method.
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