First things first -- I'm still a little confused on what 2s2p and 2s1p look like. I was good to go on 2s2p, but then 2s1p kinda threw me for a loop. 2s2p is a total of 4 cells in two groups where the two cells in each group are in parallel, and then the two groups are wired in series -- is this correct? If so, is 2s1p a total of 4 cells in two groups where the two cells in each group are connected in series, and then the two groups are in parallel?

Am I getting this right?

So what is the advantage to one over the other? It seems that if each cell was M mah and V voltage, you'd have 2M mah capacity and 2V voltage either way... so what am I missing?

It would seem that a major disadvantage of wiring lipos cells in parallel is the loss of the ability to balance the individual cells in a parallel group. Why would you go with 2s2p ever?

I have a feeling I'm missing something.

The 2S2P info is correct, but for let say a 2S1P 5000mAh pack, you'll have 2 5000mAh cells in series and not 4. For example, in my 8th scale truggy I run 2 3300-3700mAh 4S1P packs wired in series to give me 8S1P, but I can have the packs wired in parellel and have it 4S2P which will increase the capacity.

So 2s1p is just two cells and zero parallel connections/groups? Why do they put 1p?

The reason you sometimes see higher than 1 parallel is just packaging. If you don't have cells just the right size to fill a battery compartment, but have cells that are right at half the desired size, bingo just double them up in parallel.

No worry about balancing, the cells in parallel will continuously balance with each other. All the time whether being used or not (within a single assembled pack anyway). Normal balancing still recommended for the series portion.

The only concern is when assembling the pack, should have all the cells at the same voltage. Any difference, and the balancing will happen very quickly, could cause some high currents.

Good question and I have no clue lol.

It would seem to me that placing cells in parallel would prevent individual cell balancing. Let's say you have two cells in parallel -- you can only treat it as one cell. At that point, if one of those cells gets slightly off, you cannot correct it without pushing the other cell the wrong direction. Let's say you have two cells in parallel, one at 100%, the other at 95%. Not only can the charger not tell which cell is which, if you were to somehow determine you needed to drop one down, it would take them both down because they are connected together in parallel.

See what I'm saying?

Seems to me that it would always be preferable to have better single cells in series than to have cells in parallel at all.

Exactly thatīs right. But as it was said before, sometimes there is no other choice for a manufacturer for using cells in parallel. As long as these are high quality cells with little variance (or selected) this seems not to be a big problem but from my view there should be no more than 2 cells paralleled. I know there are several Lipos out there using 3 or more cells paralleled and this is what I wonīt buy. Any serious manufacturer will print the configuration at the specsheet so you will know.

Dave is also right cells in parallel will balance each other. This isnīt just for Lipos but for batteries in general. But this also is a reason why you shouldnīt parallel batteries with different or unknown charging state. If there is a big difference there will be high current flow between these what could mean even destroying them if the difference is big enough.

Well thatīs just an usual technical specification.
See it that way, if you have a Lipo what is just rated as 2S any informed guy will know thatīs a 7.4V Lipo. But this informed guy also knows there are Lipo packs using just one or maybe more cells in parallel but as there is just 2S spec he wonīt know anything more on this.
If there is the 2S1P this then means there are just 2 cells in series connected but no additional cells in parallel. Of course you also could say 2S0P with 0P meaning no additional cells, but this looks a little weird in compare to 2S1P from my view.