There are different type of for loops, What do they do?

I saw a simple for loop which an example would be this:

for i = 1, 3 do
    print(i)
end

Also a more complex for loop that I found from the wiki:

local Players = game:GetService("Players")

function onPlayerAdded(player)
     print(player.Name .. " has entered the game")
end

--When a player joins, call the onPlayerAdded function
Players.PlayerAdded:connect(onPlayerAdded)

--Call onPlayerAdded for each player already in the game
for _,player in pairs(Players:GetPlayers()) do
     onPlayerAdded(player)
end

I don't really know what these do.