Yes, it can be very confusing!
There are two types of drives Fixed and Auto.
A fixed speed drive will only work in a system which is the same or have a faster interface. As an example you have a fixed SATA II (3.0 Gb/s) drive it will work in a SATA II system or in a SATA III (6.0 Gb/s) system. It will not work reliably in a SATA I (1.5 Gb/s) system.
Unlike a fixed speed drive an auto drive can work in different systems as it has the ability to match the systems SATA port speed. But be careful here! Not all auto drives run in SATA I systems! Many drives today have dropped the slower SATA speed and only work in SATA II & SATA III systems.
So how do you know what the drive is fixed or auto? The only way is to review the given drives spec sheet. Here's an example of a fixed SATA speed drive: Seagate 2.5" standard HD. Now lets look at a auto sense drive: Seagate 2.5" SSHD. Now lets look at the 'Interface' line note the standard HD drive only lists 6.0 Gb/s and the SSHD model offers all three data rates.
The spec sheet I've listed here for the SSHD is for the older SSHD model. Seagate just intro'ed a newer drive series called FireCuda SSHD note the 'Interface' line is now only listing 6.0 Gb/s! Many drive makers are dropping not only SATA I support but also SATA II. You do need to be careful what drive you get now when upgrading these older systems.
FYI: Western Digital only offers SATA III (6.0 Gb/s) HD drives, Seagate was the HD holdout,
As to your question on getting a SSD for your MacBook: Again, you need to review the given drives spec sheet. If its not written don't take anyones word.
Many SSD vendors today only offer dual SATA speed support in their auto drives (3.0 Gb/s & 6.0 Gb/s). You can find lots of fixed SATA III (6.0 Gb/s) drives which won't work in your system.
Samsung is the only holdout offering a full auto speed SSD (1.5, 3.0 & 6.0 Gb's support). Here's the spec sheet of their cheaper SSD models: Samsung 750 EVO