Lync 2010 SDK is the latest Client-side SDK for Lync 2010 (doesn't work against previous versions). Its a fully managed (i.e. .Net) API, and allows you to build apps that automate the running instance of Lync to perform tasks, e.g. display presence and start a conversation from your LoB app (Automation mode). It also allows you to build your own custom clients to completely replace the Lync UI (UI Supression mode).
Unified Communications Managed API 3.0 (UCMA) is the latest server-side SDK for Lync 2010 (doesn't work against previous versions) - its also a fully managed API, and allows you to build applications that register to Lync as endpoints. This is useful for building bots, for example, a stock ticker bot that IMs people when a stock price goes over a certain threshold. You can also build call routing and IVR (interactive voice response) apps with this, and it can use Windows Workflow Foundation to reduce the amount of code you have to write.
Lync Server 2010 SDK is another server-side SDK, but rather than creating endpoints as per UCMA, this allows you to write filters that plug in to a Front End server. A filter allows you to inspect SIP traffic as it passes through the server, so its useful for building e.g. archiving solutions.
Unified Communications Client API (UCCA) is a client side API that allows you to build custom endpoints - e.g. Lync client replacements. This is best avoided, as it is COM based, so you have a lot more code to write, and need to be very careful about memory leaks. Its recommended to use Lync SDK in UI Supression mode, instead of this API. One benefit it does have though, is it works against Communicator 2007, Communicator 2007 R2 and Lync.
If you're developing against Lync and no previous versions, then Lync SDK (client) and UCMA 3.0 (server) should give you what you need. Otherwise, this article here talks about the previous SDKs in a bit more detail
Edit: I've expanded on this in a blog post
