Generally, if the peak panel output current is low enough compared to the battery
size, they can be directly connected and will act as a basic trickle charger. Anything more - which to me means anything useful - would need a controller of some sort.
Controllers can be as simple as a two-setpoint on/off device like those used in basic "automatic" battery chargers; this is what I have, and it just turns off when the battery gets to some set voltage (e.g. 14.2 V), and on again when the battery drops to something lower (near 12V).
Better controllers act more like "smart" battery chargers, controlling the current to charge the battery appropriately. These are usually pulse-width modulation (PWM) designs. Some FiberglassRV members have these.
The best controllers convert between different voltage levels for the panel and battery, controlling the panel for maximum output, independent of battery voltage. These are the maximum power point tracking (MPPT) designs. I've only read about them on solar power