You will often hear these referred to as "palette knives" although
strictly speaking a palette was designed for mixing paint on the
palette. These knives come in a variety of shapes and are very
versatile as painting implements. I prefer ones that are not too
small (the work tends to look "finnicky") and not too narrow (they
don't hold enough paint).
At first, painting with knives can feel a little like trying to write
with boxing gloves on, but they are great for building up textures
and are conducive to bold expressive work. Colours also tend to
stay "cleaner" because, unlike brushes, you get a completely clean
blade with a single wipe with a cloth.