been researching internet plans in my area, and I have come across one question. When an ISP advertises a certain mbps, what is that referring to...upload speed, download speed?
The most prevalent one is download speed, although a lot of ISP have started to advertise both (so if you see something like 50/10 then it's 50 mbit down and 10 mbit up).
Also, FWIW, Verizon is in the process of upgrading their FIOS network to allow same speed uploads as what you pay for downloads. When it's complete and what order they are doing it in - I have no idea, but you may be in a lucky area and already have it.
Most cable and DSL providers have crappy upload speeds, some you can find have a 100Mbps download speed but only 5-10Mbps upload. Lots of fiber providers offer symmetrical speeds, so you'd likely see them advertised as 25/25, 50/50, etc.