NPT is the most common pipe thread in the US. It is also known as black iron pipe thread. This is a tapered thread.
NPS is another American standard for a different pipe thread that is not common. This is not tapered.
The US National Pipe standards also have several other variants like dry seal, railing, etc.
BSP is a common pipe thread in Europe. It has straight threads with no taper. As with most British threads, the thread form itself is different from what we have here in the states. Fortunately, the pitch of the thread is usually the same as NPT, so I can often make a BSP female into NPT with a small amount of effort & a tap.
If you want to understand all the details of many different threads, then you can look them up in a book called the Machinery's Handbook, but it's a heavy read.
http://allaboutmetallurgy.com/wp/wp-content/uploads/2016/12/Machinery_s_Handbook_29th_Edition.pdf
If you just want some basic info, you can take a quick gander in these few places -
https://www.ralstoninst.com/news/story/the-difference-between-npt-bspp-and-bspt-seals
https://www.usplastic.com/knowledgebase/article.aspx?contentkey=998
https://en.wikipedia.org/wiki/National_pipe_thread
I normally use tape on my NPT threads, unless it is for gas or fuel, then I use the appropriate dope. I know more than one guy who swears by using both tape & dope all the time for everything. There are different formulations of dope for different materials.
I seal my thru-hulls with 3M 5200 if I want a permanent seal. If I want something removable, I cut a gasket out of the appropriate thickness of a suitable rubber gasket material & coat it with Dow Corning 111. In some cases, an O-ring is also a good choice.