Can somone tell me the diff. between a 2s2p and a 2s1p lipo battery.

A 2s1p is just a 2s pack ie 2 cells in series to give you 7.4v at what ever the mah rating is on the pack. If it is rated at 2200mah then each cell is rated at 2200mah.

A 2s2p pack like this...

[link]http://www.hobbyking.com/hobbyking/store/__18605__ZIPPY_Flightmax_5400mah_2S2P_35C_Hardcase _Car_Lipoly.html[/link]

is made up of two sets of 2s cells in parallel. But each cell is only rated at half the total pack mah.
That is to say, there is a 2s 2700mah pair of cells connected in parallel with another 2s 2700mah pair of cells...2s2p 5400mah.

I think I've even confused myself. But that's it.

With just that information a :
2S1P is a pack with (2) individual cells wired in series
2S2P is a pack with (2) sets of individual cells wired in series and then each set of (2) series wired cells are then wired in parallel.

If each cell in each pack has the same "C" rating and the same MAH rating then the 2S2P pack will provide double the capacity (MAH rating and "c" rating) versus the 2S1P pack. Example using a 3.7v 2200mah 20C cell: the 2S1P pack would equal 7.4v 2200mah 20C and the 2S2P pack would equal 7.4v 4400mah 40C.
If the packs are made to have equal "C" ratings and MAH ratings then the main difference between a 2S1P and a 2S2P would be weight as the 2S2P would be heavier. Example : a 2S1P pack rated at 7.4v 2200mah 20C would need (2) 3.7v 2200mah 20C cells and a 2S2P pack rated at 7.4v 2200mah 20C would need (4) 3.7 v 1100mah 10C cells.