I got another 2 questions that are probably stupid but i just want to clear them up- What type of interfaces are there and what is the best type to get? and what is a chache?
All computers use SATA nowadays. SATA connectors have an L-shaped hole.
There are differences within the SATA standard, also.
There is SATA-I, or SATA-150. It transfers data at a theoretical maximum of 150MB/s.
There is SATA-II, or SATA-300. It transfers data at a theoretical maximum of 300MB/s. This is the configuration which most hard drives have.
There is SATA-III, or SATA-600. It transfers data at a theoretical maximum of 600MB/s. Not many hard drives have this. This is usually used with SSDs, which are fast enough to actually take advantage of the maximum speed.
The 2011 MacBook Pros support up to SATA-III on the hard drive slot and SATA-II on the optical slot, if you're interested in OptiBay. Therefore, it is not wise to put a solid state drive in the OptiBay slot, because the interface will bottleneck its performance.
If you want a decent hard drive, then get one with at least SATA-II.
Cache is a short-term solid state memory reserved for the hard drive. It serves as a queue for operations, like a to-do list. It ranges from 8MB to 64MB. Usually, the bigger the cache, the better the drive, because a bigger cache means the drive does not become overloaded easily. In most cases, you could consider it as RAM for the hard drive.
However, this is not strictly true. Hybrid SSDs, which have a large amount of "cache', can use this to their advantage. Since the cache is made of SSD chips, it is significantly faster than the hard drive itself, so if they store frequently used files in the cache, then they can load it faster. The Momentus XT has a cache size of 4GB and uses this technique to speed operations up.
SSDs don't have caches, since the entire drive is practically made of cache chips.