There is a lot of terminology floating around that.
First, the basics: There are word forms occurring in real life. A word form belongs to a lemma that is a canonical word form relating to some other forms (e.g., for the lemma "have" there are the word forms "have", "having", "has", and "had"). Then, there is a stem that is generated off a word form by cutting some affixes. All those terms relate to basic morphology (there is not a lot of it left in English, other languages have a much richer morphology). Note that one lemma can have several (more or less obviously) related stems, like Latin scribere, scribo, scripsi, scriptum exhibiting three stems (present, perfect, and supine stem).
Morphemes come into play when you consider semantics. Derivational morphology and composition are means of creating word forms (and even stems) containing more than one morpheme. A word form containing only one morpheme is called monomorphemic. With two morphemes, it is dimorphemic and so on. As an opposite of monomorphemic, the term polymorphemic can be used.
You can also put your focus on word formation, than you find that a word is suffixed (only rarely suffigated, has a suffix attached), prefixed (with a prefix), infixed (with an infix), or just affixed (with an unspecified affix).