The major difference between the two is that botulinum toxin is a neurotoxin while dermal fillers are not.
A neurotoxin such as Botulinum toxin Type A is a substance that interrupts neurological communication between specific nerves and muscles thus temporarily preventing muscles from contracting and creating wrinkles.
In contrast, rather than blocking communication between nerves, HA dermal fillers will restore age-related volume loss and improve the appearance of facial wrinkles by supporting and strengthening skin directly under the wrinkles with hyaluronic acid molecules (the same ones naturally found within skin tissue). These molecules attract water back into cells underneath your skin's surface so that they plump up over time thanks to the extra hydration boost. They also help keep moisture locked inside.