The two explanations given are accurate and quite clear, but for the newbies, let me expand a little bit...
As fast_rx said, PocketFrog is a library of graphics functions. Things like drawing circies, blitting bitmaps, rotating surfaces, collision detection, etc. All of that is contained in PocketFrog (any version, PocketHAL edition or otherwise).
PocketHAL, as Presto said, is a replacement for GAPI. In other words, PocketHAL will give you a pointer to an area of memory where you write out graphics for display. When using PocketHAL, *YOU* will have to write all the functions named above, and whatever else you want.
PocketFrog 0.8.x is the best of both worlds... It's the library component of PocketFrog, but it uses PocketHAL instead of GAPI under the covers. If you are using PocketFrog, you will 98% of the time not care (from a programming standpoint) whether it's using GAPI or PocketHAL, the code you write will be the same. You of course WILL care in terms of performance because PocketHAL is so much faster than GAPI, and in some situations you might care from a code standpoint, like direct surface nanipulations (which should be a lot easier with PocketHAL because it's linear).
I don't think there's any reason to be using PocketFrog pre-0.8.x. The only reason I can think of is if you want a completely free product (PocketFrog based on PocketHAL is "donateware", more or less... Not sure if Thierry has a different name for it
). If you are interested in writing your own library from scratch, PocketHAL will make that task easier and help maximize potential performance.
...and so I said to Mr. Gates: "$640 billion should be enough for anyone!"