This may be a stupid question but what's the difference between a long and short block ie what does the short block include

 

with or without heads basically, short block is just the block itself, long block is with heads

 

And heads are? For instence would the short block contain pistons etc

 

Yes.

In both cases it refers to a fully built-up engine i.e. pistons+rings/gudgeon pins+circlips/rods/crankshaft/main and big-end bearings/oilpump/sump/waterpump, etc.

Just that a long engine comes with fully assembled heads/headgaskets/headstuds attached to the block; a short engine doesn't.

 

Heads = Cylider heads

 

The next question is then does a sti short block come with forged pistons

 

Yes, an OEM STi does have forged pistons (at least by name ). They're not as 'good' as the myriad aftermarket brands, touted as being highpower-capable though.

That said, Classic STi forged pistons should be good for up to c.400bhp. New Age STi, for about 450-470, I guess. (2.5 STi is wise to stop at 430-450).