Synchro Tuner Monsters are simply Synchro Monsters that are also listed as Tuners, similar to Toon monster, Union monster or other secondary "Archetypes". While cards of this type (as in having Tuner printed on them) were officially introduced in "Starstrike Blast," it was already possible to simply make/treat Synchro Monsters as Tuners (such as with Lightwave Tuning or Psychic Tuning). Synchro Tuners in particular however are required for a new method of Synchro Summoning called "Accel Synchro Summoning", which is used to bring out the namesake "Accel Synchros", has well as Hyper Synchros for YCM. (which will accept any Synchro Tuner, whether it was a "printed" or "altered" monster). Otherwise, these are still simply both Synchro Monsters and Tuner Monsters with all the rules and conditions that normally apply to both respective types of Monsters.
That said, in the OCG/TCG, (printed) Synchro Tuners, except Life Stream Dragon, also have an additional condition allowing the player to conduct a Synchro Summon during their opponent's Main Phase as long as they are used as one of the Synchro Materials. In the anime, this condition was actually part of the Accel Synchro Monsters themselves, but with this change in the real life version, it downgrades them to normal Synchros with restrictions and upgrades any Synchro that would use a (printed) Synchro Tuner into having the same effect that was originally reserved for Accel Synchros.