Yes.
Different classes work differently. Druids (as well as Wizards and Clerics) have to memorise spells via the spellbook (

. You chose which spells you want and rest, and then you can cast them the next day.
Sorcerers (and Bards) cast spells spontaneously. This means that they have so many known spells of each level, and can cast any spell of each level X number of times. For example, a Sorcerer who knew, say, Burning Hands and Mage Armour could cast Burning Hands X times, Mage Armour X times, or a combination. He probably knows other L1 spells he could cast instead. However, he can't cast Melf's Acid Arrow instead of those spells, because that's a 2nd level spell, and he has a different number of 2nd level spells per day.
This makes Sorcerers more flexible during an encounter, because you don't need to decide in advance exactly which spells you want. They also get more spells per day. The flip-side is that they are very limited in their known spells. Whereas a Wizard could memorize one each of a vast menu of spells, a Sorcerer might be stuck with only one or two options, even if he can cast them lots of times and can use the most useful one for each situation.
Warlocks, incidentally, are different again, in that they can use their Invocations as many times as they want per day. They're completely unlimited, but they get even less of them than Sorcerers get spells. As such you're stuck firing off the same things over and over (Warlocks make decent archer or blaster types).