what is a cog in discord.py code example

Example 1: how to make a cog discord.py

from discord.ext import commands

class Test_Cog(commands.Cog):
	def __init__(self, bot):
      self.bot = bot # defining bot as global var in class
      
	@commands.Cog.listener() # this is a decorator for events/listeners
    async def on_ready(self):
      print('Bot is ready!.')
      
	@commands.command() # this is for making a command
    async def ping(self, ctx):
		await ctx.send(f'Pong! {round(self.bot.latency * 1000)}')
        
def setup(bot): # a extension must have a setup function
	bot.add_cog(Test_Cog(bot)) # adding a cog

Example 2: discord.py cog classes

class Test(commands.cog):

    def __init__(self, client):
        self.client = client
        self._last_member = None