There are actually two Xeon E5 sockets and the E5 Xeons are available in single-CPU-only, dual-processor-capable, and quad-processor capable units.
There is LGA1356 which is for the single-CPU-only E5-14xx parts and the dual-CPU-capable E5-24xx parts. LGA1356 is very similar to the Xeon 5500/5600 and previous generation Core i7 LGA1366 socket. There is also LGA2011 for the single-CPU-only Xeon E5-16xx parts, dual-CPU-capable E5-26xx parts, and the quad-CPU-capable E5-46xx parts as well as the Sandy Bridge and newer "big" Core i7s.
LGA1356 and LGA2011 Xeons differ in the following ways:
1. Each LGA1356 socket provides 24 PCIe lanes from the CPU as opposed to 40 lanes per LGA2011 CPU.
2. Each LGA1356 socket supports 3 channels of memory and each LGA2011 socket supports four channels of memory. They use the exact same kinds and speeds of memory. The extra channel in the LGA2011 parts increases memory capacity by a third over an LGA1356 setup.
3. LGA1356 CPUs are limited to a maximum of 95 watts while LGA2011 CPUs are offered in up to 150 watt guise. This means that LGA2011 CPUs will in general be higher clocked but are otherwise offered in identical in core counts/cache amounts as LGA1356 CPUs.
4. LGA2011 has provisions for a second QPI link to enable (inefficient box-topology) four-socket operation, whereas LGA1356 sockets only can route one QPI link for a maximum of two-socket operation. QPI speeds and widths are identical between the two sockets.
5. There is a much better selection of LGA2011 Xeon parts compared to LGA1356 Xeons. There are several E5-1600s vs. two E5-1400s and about half as many again E5-2600s vs. E5-2400s.
Note that there are actually FOUR currently used Xeon sockets. There is LGA1150 for the Haswell single-CPU-only Xeon E3 V3s based on the desktop Haswell Core i3/i5/i7 models. There is also LGA1567 for the E7 Xeons. These are old Westmere-based parts available in 2, 4, and 8 socket capable parts with up to 10 cores. You likely will never encounter an E7 Xeon unless you are working somewhere with a bunch of 4 or 8 socket servers.
I find it hard to recommend Xeon workstations right now as there are very few E5-26xx Xeons which are reasonably priced but not crippled. The most sane of the bunch would be the 2.1-2.6 GHz six-core E5-2620v2 at $410 each and possibly the 2.6-3.1 GHz six-core E5-2630v2 at $612 each. Most of the E5s are over a thousand bucks and the really fast ones are over $2600- each. Ouch. It's kind of hard from an enthusiast perspective to pay that much for one CPU when you can buy a quad G34 Opteron board with four decent 12-core Opterons like the 6348 and absolutely kill the single Xeon in multithreaded tasks. Sure, the Xeon will be faster in single-threaded stuff but if that's your forte, get an i5-4670K, spend a lot less, go faster, and call it a day.