Derivational suffixes, such as "-ness" or "-ful," can change a word's basic meaning, often converting a word from one part of speech to another. Inflectional or grammatical suffixes, such as "-s" or "-ed," generally do not change the meaning of the word but signify a change in number or tense.
Derivational suffixes can change the meaning of a word quite drastically. Derivational suffixes often indicate the relationship between the derivative word and the original. For example, the suffix "-er" often means "someone who does something." Adding "-er" to the verb "sing" creates the noun "singer," meaning "one who sings." Other common derivational suffixes include "-ment," which often indicates condition or quality, as in "enjoyment", and "-ism," which usually indicates a set of beliefs, as in "Catholicism."
Inflectional suffixes generally indicate either the pluralization of a noun or a certain conjugation of a verb. For example, "-s" usually denotes a plural, as in the word "dogs," and "-ed" generally indicates the past tense, as in the word "walked."