Macro Tube / extension tube , pretty much the same thing .. ( I think they are called Macro extension tubes to be more correct )
There are many different types ... You have Macro extension tubes > Their only function is to move the lens away from the body , there by increasing magnification and changing focus ( how far the lens can focus )
Then you have macro adapters , these can be lens specific , as well they most likely offer some sort of multiplier 2x or 3x or some such , there by changing a 50mm lens into a 100mm or 150mm lens ..
I just got a 3x multiplier Macro adapter off ebay a week ago ( net here yet )
I currently own 2 of the 2x adapters ....
Currently the cheapest Macro option ( offering decent results ) is a good 50mm lens and a Macro tube .. If you have a lens , then all you need is the tubes ...
I will say it here and now : IF YOU DONT OWN A ( 100mm F4 ) MACRO LENS ( pentax ) , either the M or A version , save your pennies and buy one soon , because the prices are going up and I have noticed a steady decline in supply of these old lenses ...
These lenses are darn good , and I have seen decent ones this year starting around $80 to $90 ( selling ) , but recently the prices have jumped , as has availability ( gone down )
Maybe next year things will improve ? . but prices have gone up across the board !
M42 ? I have no interest in M42 , I recommend avoiding them for Macro , and I speak from experience ( owning several M42 lenses ) .. Let those who love M42 have them ...
You want a K mount lens ...
Buy that 50mm and 100mm ASAP ...
You dont need f2.8 or lower , Macro is shot around F16 or better ...
And the Pentax A 50mm F2 is a bargain buy ATM , though some would say the bargain is the f1.8 55mm