The family Colubridae is a family of snakes -- one might even say the family of snakes, as it contains nearly 70% of all snake species. It contains about 12 subfamilies, 304 genera, and approximately 2,000 species. Unfortunately, many of these genera should not properly be grouped together. Colubridae is used as a catch-all category for those genera that have not been properly classified yet. Theoretically, this family will eventually be whittled down substantially, and many species will be split off into more appropriate clades.
For obvious reasons, there are no strict defining features of this family -- nor even any general rules of thumb. Colubridae includes venomous and non-venomous species, oviparous and viviparous species, and they appear on every continent (excluding Antarctica).
Members of Colubridae are sometimes referred to as colubrids. Snake identification guides will often use this word as if it means something specific; rest assured, it means nothing other than "a snake that has not been categorized properly yet".