They have the same CPU socket but use a different chipset, which may or may not limit CPU compatibility (Intel are famous for keeping the same socket and still breaking compatibility, but I'm not familiar with their recent stuff).
Fake edit: I've looked on Gigabyte's website, and the B85 one "Supports 4th Generation Intel® Core™ processors" while the Z97 one "Supports 4th and 5th Generation Intel® Core™ processors". See what I mean?
They both support up to 32GB of DDR3 ram at 1333 / 1600 MHz, but the Z97 mentions ram overclocking up to 3100 MHz in its specs, if you're into that kind of thing.
They both have one PCI-e 3.0 x16 socket for a graphics card, but the secondary card sockets are different - the B85 has two PCI-e x1 connectors for most modern non-graphics cards (e.g. WiFi) but it can't take any old PCI cards at all, nor a PCI-e x4 high-end card unless you give up the graphics socket. The Z97 has two old-school PCI sockets, plus a secondary PCI-e x16 socket that's only really wired for x4 (and is only half the speed on top of that, PCI-e 2.0 vs 3.0), which is still just enough to run some secondary graphics cards (I wouldn't put a high-end card in it) but does support PCI-e x4 high-end non-graphics cards, if you need any, and will still take a standard PCI-e x1 WiFi card etc.
They both have 6 sata ports for drives, but the Z97 has all 6 running at sata3 / 6Gbps, whereas the B85 has two at the older sata2 / 3Gbps. This probably doesn't matter unless you're putting 6 SSDs in your PC.
They both have 6 USB sockets on the back, but the Z97 has four high-speed USB 3.0 sockets (for high-speed external drives mostly) where the B85 only has two. I'll be honest, you probably don't need more than two on the back, USB 3.0 devices are still relatively rare. They both have internal headers for two USB 3.0 connectors (best used for the front of the PC, but you can get brackets for the back too), plus a bunch of extra headers for extra USB2.0 connectors if you need them.
That should about explain it.