Tidal Locking is the locking of one of two orbiting bodies so the same side always faces the other body. The body's rotation period matches its orbital period. It is caused by the effects of the Tidal Force due to a body's orbiting peer on its rotation, which tend to speed or slow the rotation toward the tidally locked state.
Deviation from spherical symmetry (e.g., mountains) can contribute to this type of locking, but even if the planet is, for all intents and purposes, spherically symmetric, tides within a "solid" planet can result in such locking. The difference in gravitational pull on the near and far sides of the body stretches it to create an asymmetry deviant from a perfect sphere, and if the body's rotation forces the migration of this asymmetry around the body, a drag on the rotation results from the friction of the constant reshaping of the body.
Either the larger or smaller body can be so-locked but it is typical for the smaller body to be locked.
Venus is not tidally locked to the Sun: it has a solar day of 116.75 Earth days whereas tidal locking would result in endless days.
Three Dimensional Model