A suffix is a letter or group of letters affixed to the end of a word to create a different word. Some suffixes are single letters; for example, in the word floors, -s is a suffix indicating that the noun, floor, is plural. Others are multiple letters; for instance, in the word brightest, the suffix -est makes the adjective, bright, superlative.
There are many ways to categorize suffixes, but the most common makes a distinction between inflectional suffixes and derivational ones. An inflectional suffix adds information to the root word without altering the root’s linguistic function. For example, the -er in the adjective looser is inflectional because loose and looser are both adjectives that function more or less the same. The suffix -hood, in contrast, is derivational because it combines with its stem to create a new word—for example, the noun likelihood from the adjective likely.
Most suffixes can be attached to their stems without a hyphen. For example, while spell-check might catch words like documentable, youthism, kittenesque, and buildingless, these are perfectly good words with easily understood meanings (especially in context).