Explicit and Implicit Hydrogens: Taking liberties with valence
The field of cheminformatics has two concepts of implicit vs.
explicit when it comes to talking about hydrogen atoms.  The
first concept is at the internal data structure level where
explicit hydrogens are each stored as an atom object (like any
other element) vs. implicit hydrogens that exist only as a
hydrogen count field associated with a parent atom.  The second
concept is at the file format or line notation representation
level where the number of hydrogens or valence is either
explicitly specified vs. encodings where the hydrogen count is
implicitly defined by some (often undocumented) valence model. 
The existence of these two related but independent concepts is a
recent personal realization after discovering what I’d
previously called an “explicit hydrogen” differed from what
OpenBabel or RDKit call an explicit hydrogen.  To demonstrate
the difference consider the following three SMILES for methane: 
“C”, “[CH4]” and “[H][C]([H])([H])[H]“