There is a great deal of good information in this thread but unfortunately some misinformation as well. I maintain over 20 batteries in various applications from tractors, buldozers, personal watercraft, jet, ski, and pontoon boats, classic cars, forklifts, stake trucks, garden tractors, standby generators to name a few.
There are obviously different types for different applications but by far the most universal is the AGM (Absorbed Glass Mat) also frequently called a gell cell in smaller batteries. Since they contain a gell instead of liquid they are sometimes referred to as spill proof. They are much more resistant to shock and vibration making them far the perfect choice for pwc's, quads, garden tractors etc. AGM's also have a very slow loss of charge when in storage and therefore survive long periods of neglect during off season use such as classic cars, boats, and other seldom used items like my standby generators. I have a 16 year old AGM battery that is only occasionally used for the winch on my car trailer that still works perfectly. I think that definitely disputes the comment that they have a poor shelf life. I think you can understand why if at all possible I buy AGM's.
It was previously explained that most vehicles have a continuous small draw to maintain clocks or computer memory etc. A solution to that is to disconnect one of the battery cables during storage. Several disconnect devices are available ranging from high capacity switches to terminal adapters with knobs for ease of use. Most of the maintainer/chargers supply enough trickle charge current to overcome that loss. Regardless, it is always a good idea to disconnect a battery when in long term storage.
Many brands of maintainer/chargers are available and most work by turning on if the battery voltage drops below 13.2 volts and turns off when voltage goes above 13.8. That is the actual voltage of what we refer to as a 12 volt battery. When batteries are frequently cycled such as seen in a typical vehicle, the alternator charges to a slightly higher voltage of 14.2 volts or more initially to fully recharge a battery after a start. Vehicle computers in newer vehicles drop the voltage down to the 13.2 range after an extended period of driving to prolong battery life.
I have several different brand maintainers. About 10 of them are Schumacher 1.5 amp charger/ maintainers that were about the only good ones available 20 years ago. Numerous batteries have lasted 10-15 years using those. During the last 5 years or so I purchased several Harbor Freight 1.5 amp charger/maintainers that are very affordable and seem to do the same job. I don't find them to be any different than the more expensive Battery Tender brand that I also own. DONT confuse them with the little trickle battery maintainers sold by H.F. and others that look like a cell phone charger. There are newer maintainers on the market known as high frequency chargers that even automatically adjust to the correct 6/12/24 voltage as required. They are advertised as necessary for the new expensive lithium batteries. I recently purchased a couple of them but don't have enough experience to comment on those yet.