You know I do this broadband wireless stuff for a living (well sort of anyway) and even I find myself confused by the terminology people are using to subdivide micro relays of various types.
Maybe it is because I tend to think we already had enough terms for this stuff because after all femtocells are really just cellular home wireless networks used for residential purposes. The difference I suppose is that the femtocell is not specifically geared for public access and is based on licensed spectrum and of course it would be carrier specific (at least until this open network concept blows that up).
So first some quick definitions, the femtocell is the smallest of the three. It is a home gateway essentially, not too different in concept from a Wi-Fi home wireless router. The unit connects to an existing DSL or Cable or other broadband connection and then by receiving IP-based cellular traffic re-broadcasts cellular service for the cellular carrier you subscribe to within your home. This means the re-broadcast would be in licensed frequencies and would only work for phones that use the technology of the carrier. So for example an AT&T femtocell would not work for either Verizon or Sprint. There are some vendors that are offering dual Wi-Fi and cellular broadcast but that is rare.
Now why would this be good? Well, for me specifically, I don't get great cellular service at my home office. My house is in a depression and I have low-e glass windows that attenuate signal anyway. Plus it seems like calls drop a lot more than they used to. And quality is not great. So, I still have to use a landline. I would love to give up that landline. I'm tired of paying two bills and have been thinking of not only changing cellular carriers but going all IP and using Skype with a gateway to use my old landline phones. Why pay two bills?
So anyways, this is all part of the fixed-mobile convergence everybody is talking about but which never seems to happen. Will it? Well, maybe as the concept is a good one and if service is better why switch which should help carriers with churn. The main problem is that the gear is expensive and even with cellular carriers subsidizing costs the units are not super cheap as yet. Plus can the average homeowner install it and what about frequency channelization coordination for a licensed carrier?
Okay so the picocell is the next size up (bigger) and the term is usually used to describe in-building wireless re-broadcast to a single floor or a building or block of suites or perhaps in a train, train station, airport terminal or other small locale. It is most analogous to a WiFi hotspot in concept in that multiple customers could access the same picocell transmitter. This type of device is not intended to be installed by the end user and needs physical security and access for installation and maintenance by a carrier. This is a bit of a specialized niche for wireless.
The microcell and the picocell can be interchangeable in terms sometimes as both cover a smaller area such as once again a building or group of suites. But more commonly a microcell refers to a residential neighborhood, an office complex or campus, an airport or other larger footprint deployment. How do you tell the difference? That is hard to say as the two terms are so similar in meaning in this industry context. I tend to use the term microcell myself. Either way, the microcell is closest in concept to a municipal wireless access point, but can be re-broadcast from either higher rooftop or lower locations on streetlamps. In all cases these are more cellular industry terminology than what those in the broadband wireless industry have called the things.
I hope this helps clear up any confusion---and that it added little to your confusion.