There's a nice description of how the turbolaser works in the Star Wars: Official Fact File #45. In essence, the technology uses lasers to energise the plasma which is then ejected at high speed toward the intended target.
Imperial turbolaser technology uses intensely focused lasers to
energize compact pockets of Tibanna gas until the weak molecular bonds
of the gas break down. Once that occurs, a second beam of photons is
introduced. This excites the free molecules to such a high temperature
that the electrons on the individual atoms break away and the gas
becomes plasma. These pockets of plasma are retained in a small
magnetic bottle at the base of the turbolaser barrel, until the moment
the weapon discharges.
The "turbo" prefix seems to simply refer to the fact that the gun is more powerful than a standard blaster or laser, combining the two technologies.
Moving down the canon scale, the New Essential Guide to Weapons and Technology gives us some specifics on the power of a turbolaser over a conventional laser canon. Note that that description of how it works is slightly different from the one above but there's still a laser being used to actuate the blast in the "Galven coils".
Turbolasers are two-stage supercharged laser cannons. The small
primary laser produces an energy beam that enters the turbolaser's
main actuator, where it interacts with a stream of energized blaster
gas to produce an intense blast. The energy bolt's destructive power
is incredible, and the barrel's galven coils focus the beam, providing
a range that is double or triple that of conventional laser cannons.
Turbolasers also can target planetary surfaces for devastating ground