所有这些Web平台如何实现不需要用户一次又一次登录的长时间登录会话？

I really don't get that: Some platforms have some kind of "persistent" login sessions. Like Stackoverflow and many other boards and platforms. I've logged in here a month ago. And even if I quit my browser totally and have another internet connection with different IP, I come back here with no login.I really don't get that: Some platforms have so