Why is there no ISO file when I download a psp game?

it has no ISO or CSO file in it. It always shows a folder which is named 'PSP GAME' then inside that folder are the icons of the game and its system directory and USR directory (I don't know what USR means). It is like the folder contains all the files inside the game. Also, besides the folder, there is a 'UMD data' in 'BIN file' (I also don't know what BIN means) and it can't be open because it shows that I don't have the program to run it.