Take a look at the implants on EVE Uni wiki, pyfa or the ingame market and you will notice some patterns about implants.
For one, the first 5 implant slots always affect the same attribute.
These implants are Attribute Enhancers.
If you have an implant that boosts perception it goes into the part of your brain that affects perception, called the first slot.
Likewise memory implants go in the second slot, willpower in the third slot, intelligence in the fourth slot and charisma implants go in slot five.
Next, some of the attribute enhancers come in sets with special bonuses, the implant sets.
As regular explorer I paid a bit more for attribute implants that also boost how well my brain can scan signatures with the ‘Virtue’ set. Such an implant set has a memory, intelligence, charisma etc. implant but with a small bonus on each of them as well as a set bonus for if you plug in multiple.
Each of those implants are called after Greek letters, Alpha, Beta, Gamma, Delta, Epsilon. And they’ve got a ‘final’ implant as well to boost the set even further, the ‘Omega’ implant that always goes in slot 6. Such sets can have various grades (mid-grade, low-grade, high-grade) which makes them more potent but also more expensive. Grades can be mixed for the set bonus.
There also are other implants than Omega that go in part 6 of your brain, as well as in slots 7 to 10.
The implants of 6-10 do not affect your base attributes and have no set bonuses but allow you to boost your performance in specific tasks.
Those implants are called Skill Hardwiring.
For example if you like harvesting gas you could plug in an Eifyr and Co. ‘Alchemist’ Gas Harvesting GH-803 implant. As the code on the end of the name suggests this goes in slot 8 and boosts gas cycle time by 3%. The better version which boosts cycle time by 5% has the code GH-805.
Unlike the first five attribute slots the last five numbers don’t really seem to have a physical meaning. It just makes slot X implants mutually exclusive with other implants that go into slot X. It means you cannot equip both the 3% and 5% gas harvesting modules in slot 8 and 9 to have even better cycle times.