When you subscribe, you receive two kinds of login. Both open the same channels; they just work differently in your app.
Xtream Codes
A server URL, username and password. Apps like TiviMate, IPTV Smarters and IBO Player use it to load channels, the TV guide and the movie library automatically, and keep everything up to date. This is the best choice for most people.
M3U link
A single web address that contains the playlist. It works in almost any player, including VLC on a computer. The guide may need a separate EPG link.
Which should you pick?
Use Xtream Codes whenever your app supports it. Use M3U for VLC or older players. MAG boxes use a portal URL instead.
See the setup guide for your device.