I was browsing E-bay, and I came across a AMD Opteron 16-core processor. When I took a look at the pictures, it says "ENG Sample" on the heatspreader. What does this mean?
It means it's an Engineering Sample, an early version of the processor that was made before it entered mass production for testing purposes. They aren't sold to the general public, so the original source of the CPU probably stole it from AMD. Engineering samples sometimes won't work properly on motherboards available to the general public, and may be inferior to the production chip if that particular sample had some bugs that needed to be worked out.
I was browsing E-bay, and I came across a AMD Opteron 16-core processor. When I took a look at the pictures, it says "ENG Sample" on the heatspreader. What does this mean?
It means the CPU is an engineering sample. Those are CPUs used by motherboard manufacturers and other OEMs to test motherboards, heatsinks, other components, and entire systems prior to the initial launch of the CPUs/platforms/systems. Engineering samples range from very early silicon that is very buggy to retail stepping parts. Engineering samples are not multiplier or voltage locked so that system integrators can test a wide range of CPU speeds/voltages/power draws/thermal dissipations which makes them very desirable for enthusiasts who want to be able to try to overclock an otherwise non-overclockable CPU.
There are a few specific things to know about a Socket G34 engineering sample, which yours is as it is a 16 core unit. The Bulldozer ("Interlagos") Opteron 6200 engineering samples run the gamut from B0 and early B1 stepping units which are very buggy and generally not usable with a retail motherboard to retail B2 stepping units which are essentially just unlocked retail units. You have to look at the last two digits of the product code, which will start with "ZS" and end with two numbers. "43" is a B0 stepping and is unusable. "44" is an early B1 stepping and is generally too buggy to be usable. "45" is generally a late B1 stepping and may be usable, while ones ending in "47" or "GU," are nearly always retail B2 stepping units and should work for you. Any Opteron 6300 based engineering sample will be retail C0 stepping and unless the specific CPU made is buggy should be OK as AMD nailed this stepping right on the head from the get-go.
FYI, my main computer is a quad Socket G34 AMD setup which is why I know about the G34 engineering samples. I personally only buy non-engineering-sample chips as a retail chip bought from somebody who pulled it out of a working server is unlikely to give me trouble compared to some Far East eBay seller selling some engineering sample of unknown provenance.