The difference is smarts.
A trickle charger provides a constant current all the time. It does not know whether the battery is charged or discharged.
A battery tender is smart. It will charge the battery only when it needs charged. After it's done charging it will shut off and monitor the battery state. When it sees the battery get to something like 80% (depends on the tender model) it will charge the battery back up to full and wait again.
Tenders are better but more expensive. Tenders charge the battery only when needed; this reduces the wear and tear on the battery.
Trickle chargers wear on the battery more.
Flooded lead acid batteries are strange creatures. They have a fairly high self discharge rate compared to other chemistries. This means that even sitting on a shelf these will die in 6 months to a year depending on size. They are also filled with water which gives them some strange properties.
An automotive flooded lead acid battery (12v) has 6 separate cells inside. Nominally each cell is 2.1V giving a total nominal battery voltage of 12.6v. These cells can and do fall out of balance meaning that one cell could be lower or higher than the rest. As a unbalanced battery charges, some cells will fully charge before others. The battery has an interesting mechanism to keep the fully charged cells from overcharging: it boils off the water. This isn't like a tea kettle whistling but an accelerated evaporation. When the battery is fully charged then all the cell begin to boil.
These properties allows the battery to work relatively well in an automotive charging system environment. Because vehicle have a relatively unmanaged charging system the battery holds up well only requiring occasional water top offs. Also batteries like sealed lead acid, absorbed glass mat and gel cell make bad batteries for vehicles because it is bad to take them to 100% charge.
Depending on the discharge rate of the battery a trickle charger is meant to off set the discharge rate plus some. If that plus some is large enough the battery will sit in that boil state indefinitely.
The tender will bring the battery up to full and stop charging, minimizing the time the battery spends in the boil state.
Both chargers wear on the battery by eating up charge/discharge cycles. Even though the trickle charges always keeps the battery full, the fact that it always charges wears on the battery just as much as charging and discharging the battery.