A trickle charger applies a low, steady current for as long as the cell (battery) is attached. A float charger ceases the charge when the cell if fully charged, then monitors the cell. When the cell reaches some lessened state of charge (as determined by the float charger's design, usually a lessened voltage), it will start charging again. Then the cycle repeats. Some batteries (depending on the chemical composition) and their intended usage are better suited to one style or the other.
The key difference is that a trickle charger does not prevent overcharging, which will damage batteries that are connected for extended periods of time. Float chargers avoid overcharging.
There's been ongoing confusion about trickle vs float, in part because trickle chargers were originally intended to approximate the self-discharge rate of some rechargeable batteries (such as NiCd and NiMh) and thus keep such cells fully charged. If a trickle charger is perfectly matched to a battery's self-discharge rate, things are fine. But, if overcharging occurs, it can lead to reduced cell life (best case) or rapid venting (think explosion, fire, and/or very toxic fumes - worst case).
The easy rule of thumb is that trickle chargers are okay if they won't stay connected for extended periods after the cell(s) are fully charged. When it will stay connected for long periods, a float charger is the safer and better choice.
A good source for battery info is BatteryUniversity.com, particularly
http://batteryuniversity.com/learn/arti ... arge_table and
http://batteryuniversity.com/learn/arti ... _batteries, and
http://batteryuniversity.com/learn/arti ... y_problems.
I'm not as familiar with SLA batteries as I am NiCd, NiMh, and lithiums, so I'm not sure to what extent they can be overcharged. I'm sure others on here can clear up any SLA-specific issues that I missed or got wrong.
