ok, I will try

pppoe provides you with encrypted tunnels from each client to a central server. it's an authentication method.
hotspot will force the user to log into a webpage form, before any webbrowsing will work. it's also an authentication method, but not encrypted.
mpls is a way to separate traffic and direct it through other ways, it's an alternative to routing, it assigns flags to packets
QOS means "quality of service" and means any method that improves the service quality maybe it means giving each user his own connection speed, making sure others don't take away his bandwidth. maybe it means a lot more.

oh, by the way - read all of this: http://wiki.mikrotik.com/

So somehow i made Slashdot front page today with my "Powerful Linux ISP Router Distribution" article and after everything is said and done, i realized that i just don't know RouterOS. Clearly the RB1000 gives the most performance per dollar in the entire industry with 400K pps, and RouterOS is fantastic - i've been using it for 5+ years now. Ubiquit AirMAX equipment is the best i've seen for last mile equipment, but to make use of that bandwidth i feel that i need RouterOS schooling. Will the RouterOS wiki really teach me what i need to know to fill in the gaps so i can build an enterprise-class ISP?

if you want all of RouterOS knowledge, then you need to combine your information sources. Watch some Tiktube.com videos, read some Wiki articles, maybe even attend a training or MUM. There are also some great RouterOS blogs out there (by greg sowell and butch evans, for example). Greg also has some nice tutorial videos.