What's the difference between ionization and dissociation?

Ionization is a special case of dissociation.

Dissociation is a process in which separate or split into smaller particles.

We might write the general equation as

##"A-B" → "A + B"##

For example,

##"NaCl(s)" → "Na"^+"(aq)" + "Cl"^"-""(aq)"##

In this case, the solid ##"NaCl"## consists of ##"Na"^+## and ##"Cl"^"-"## ions.

All they are doing when ##"NaCl"## dissolves is separating from each other.

When the substance already contains ions, the process is often called dissociation.

Many also dissociate in water to form ions.

For example,

##"HF(aq)" ⇌ "H"^+"(aq)" + "F"^"-""(aq)"##

When covalent compounds dissolve in water to produce ions, the dissociation process is often called ionization.

Here's a great video on ionization and dissociation.