These are the magnets that are tucked underneath the coils of your humbucker. The magnet is one of the two main factors to creating the sound of your humbucker, meaning switching it out for a different type will create a drastic change in your tone. For this reason, magnet swapping to fine tune your humbuckers is one of the most popular mods.
These magnets are 62mm x 3.2mm x 12.5mm, standard sizing should fit any humbucker that does not already have a large ceramic magnet in it already (if this is the case you'll need to replace the spacers in the humbucker for it to accommodate a smaller magnet). Large ceramic 62mm x 5mm x 12.5mm. 7 string 70mm x 5mm x 12.5mm.
Here's a short summary to help you choose which alnico grade is right for you
Strength of magnets (weakest to strongest): alnico 3, alnico 2, alnico 4, alnico 5, alnico 8, ceramic 8
The stronger the magnet, the more output it will contribute to your pickup. Roughcast and unorientated magnets will be subtlety less powerful than their standard counterparts and will have a slightly softened treble response. The large ceramic magnet will provide even more of all the attributes of the standard sized version.
Alnico 2: The vintage magnet. Popular in low output winds because of the full, sweet tone it gives. This contributes a large amount of mid range. It has a soft and musical treble response, and a bouncy, loose bass response. Perfect for anything clean or low gain. (Billy Gibbons, Slash)
Alnico 3: The lowest level of output, pretty balanced and a bit sharper than Alnico 2. Tonally similar.
Alnico 4: The PAF magnet. The magnet found more frequently in the fabled PAF era humbuckers. This magnet sits tonally between alnico 2 and 5.
Alnico 5: The most popular magnet. This magnet is found in most non-budget humbuckers. It has a very tight bass response and sharp treble. It is at home in high output winds as it can contribute more treble lost in the overwinding of the coils. The precision and articulation this magnet brings to a humbucker allows it to excel in high gain applications. Also popular in low output winds where a scooped mid is desired. (Jimmy Page, Jeff Beck, Joe Perry)
Alnico 8: Alnico 2 on steroids. The highest output alnico grade. Maintains the alnico sweetness, but combines it with the power of ceramic. EQ wise this is just like alnico 2, with a very present mid range. However, unlike alnico 2, alnico 8 has a tight bass and treble response. Frankly it has lots of everything.
Ceramic: The most powerful magnet. This magnet is used almost exclusively in high gain applications because of it's razor sharp treble. It has superb articulation and responsivity, making it perfect when lots of gain is used. The overwinding of high output pickups causes lots of treble loss, so the high level of treble of ceramic magnets puts back what is lost. The treble is often overbearing and too sharp in clean or low gain applications. (Matt Bellamy, Kirk Hammet, Dimebag)