JR pass is for shinkansen only?

please help me clarified my confusion. I read so many things about JR pass and am confuse. http://www.japan-guide.com/e/e2361.html

can i ask.... is JR pass referring to the shinkansen and must be purchased before your arrival to Japan. What about the regular Trains, say.. if i want to travel from Fukuoka to Beppu and wanted to take the limited express sonic or the yufuin no mori. Do i also have to purchase these train ticket before my arrival to japan or i can get them at hakata / Oita station? thanks for helping.

From the Japan Rail Pass page:

The pass is valid on almost all trains operated on the nationwide network of JR (Japan Railways), including shinkansen, limited express, express, rapid and local trains.

What about the regular Trains, say.. if i want to travel from Fukuoka to Beppu and wanted to take the limited express sonic or the yufuin no mori.

Regular JR trains are covered, as well, including the JR trains from Fukuoka to Beppu, such as the Sonic and Yufuin no Mori.

Do i also have to purchase these train ticket before my arrival to japan or i can get them at hakata / Oita station?

The Japan Rail Pass is your ticket! But if you want to reserve a seat, you have to make a seat reservation. In case of the Yufuin no Mori, a seat reservation is mandatory. You can make seat reservations at any JR ticket office in Japan.