A Pseudo-Trap Monster is a Trap Card that, like Trap Monsters, is treated as a monster by its own effect. Unlike Trap Monsters, however, Pseudo-Trap Monsters are not also treated as Trap Cards, meaning they do not occupy a Spell & Trap Zone, they cannot be destroyed by cards that destroy traps such as "Mystical Space Typhoon", and they cannot be negated by cards that negate traps or their effects such as "Jinzo" or "Royal Decree".
While all true Trap Monsters Special Summon themselves from the Spell & Trap Zones after activation as their main effect, most Pseudo-Trap Monsters instead behave as normal Traps when activated, having the same variety of effects as other Spells or Traps, but they also have a second effect that allows them to be Special Summoned from another location (usually the Graveyard) at certain timings.
Also differing from proper Trap Monsters, when a Pseudo-Trap Monster is flipped face-down while treated as a monster, it is set in the Monster Zone instead of a Spell & Trap Zone. If flipped face-up again after that, it will still be a monster. However, in this case, conditions that specify "If Summoned this way" will not be applied anymore.