Thank you to CTEK for this explanation.
There are many standard abbreviations of lead-acid batteries, and an explanation of what these mean follows:
The wet or flooded battery still dominates. It consists of all but freely suspended plates that are insulated from each other usually with the negative plate being sealed in a small polythene separator bag. The price factor is the main reason why this design is so prominent. Even though a normal car battery contains around 130 components, the production method has been refined and the material can be relatively basic. The result is a low price battery that offers something that is adequate to meet the needs of many people. A Maintenance Free battery, MF, is normally a wet battery where the design keeps gassing to a minimum and includes a battery box that is sealed to keep the gases in place. An ample amount of acid means the fluid can last for the entire service life of the battery without being topped up. Development has been made in three stages. Twenty years ago, the high antimony battery dominated the market where the basic element antimony was added to the grid, primarily to rationalise manufacture. The refined production method saw the introduction of hybrid batteries with the antimony on the negative plate being replaced by a calcium alloy which substantially reduced gassing and thereby water consumption.
The next stage has dominated on a wide front in recent years and is called Calcium-Calcium. This involves the antimony on both the negative and positive plates being replaced by calcium alloy. The benefits are obvious. The fluid loss of the battery is about 80 % lower than that of antimony batteries and the self-discharge is lower, i.e. they can remain unused for longer periods without losing a lot of their charge. The disadvantage is that they are more demanding when charging if they have been over-discharged. Efforts made in avoiding the gassing had a positive affect, namely that the bubbles moved about in the acid causing it to be thoroughly mixed when charging. Without these bubbles the acid can stratify at different densities, acid weights, and is quite a common phenomenon. An acid weight of 1.35 or more at the bottom and maybe 1.17 at the top when you are looking for an even weight of 1.28 may cause the battery to be affected by sulphating and increased grid corrosion despite the battery being apparently fully charged.
An entirely different way of controlling fluid loss goes under the generic name of VRLA, Valve Regulated Lead Acid. In this case the battery box is designed as a small pressure vessel with safety valves. If you confine the oxygen and hydrogen gases, they can react with each other and be reformed into water. This is called recombination and is a brilliant way of all but eliminating fluid loss. It is not possible to confine and recombine all the gas but the loss is reduced dramatically.
VRLA is available in two main designs, GEL or AGM (Vlies) which are constructed in totally different ways. A Gel battery has some elements, usually silicon compounds, added to the acid to ensure it gelatinises and thereby guarantee that no flooded acid can leak out. The oxygen gas "drills" channels in the gel from the positive to the negative plate where it meets the hydrogen gas and recombines into water. They have excellent capacity but the somewhat higher resistance in the acid means that they have limitations as starter batteries. They can withstand over-discharging very well, i.e. when the battery is drained down to 20% State Of Charge. Gel batteries are very robust and are often used in applications such as floor-cleaning machines and golf carts. When gel batteries are mentioned in a motorcycle context, this is often a misconception. It is more likely to be an AGM battery.
AGM, Absorbed Glass Mat, (Vlies) keeps the acid in place by the separator paper that consists of a fibreglass mat operating like a sponge. The capillary forces in the separator do the trick. The batteries can be constructed with extremely thin separators which keep the internal resistance low. This means that you can leverage high power from a small volume which makes it ideal as a starter battery. AGM has one drawback which is that the amount of acid is limited. All acid must be absorbed by the separator paper and when the small amount of acid has been converted into lead sulphate, this signals that the "petrol tank" is empty. To deal with this problem, AGM batteries often have a somewhat higher acid weight. This means that an AGM battery can, and often should, be charged using a voltage that is a little higher.
VRLA batteries are very sensitive to "unclean current", i.e. high current ripple, see link. For this reason, traditional transformer chargers should not be used.
Car Battery conditioners will keep most car batteries in tip top condition throughout their lives.