Approximants are speech sounds that could be regarded as intermediate between vowels and typical consonants. In the articulation of approximants, articulatory organs produce a narrowing of the vocal tract, but leave enough space for air to flow without audible turbulence -- approximants are therefore more open than fricatives. This class of sounds includes l-like lateral approximants (e.g. different variants of English [l]), several r-like rhotic sounds (e.g. [r\\'] as in red), and so-called semivowels, like [j] or [w] as in English yes and well.
Palatal semivowels correspond to front vowels, velar semivowels to back vowels, and labialized semivowels to rounded vowels. They are typically briefer, less stable and often closer than the corresponding vowels.