The determination of relationship terms -- what is X called -- is a task best suited to linguists and anthropologists, who ask informants to describe what is happening in their own culture. Software exists to help in collecting these terms, and matching them against known libraries of kinship terms in other cultures -- one such tool is SIL's SILKin.
It is far more likely that hobbyist family historians and genealogists will be performing the opposite task -- finding a relationship term in a document and having to puzzle out what relationship was meant. In that case, it is important to remember that the meaning of kinship terms -- like the meaning of all words -- changes over time, and also varies depending on geographical location.
Calling someone "uncle" or "aunt" in the southern United States might not indicate a person is a blood relative -- it could be a courtesy title for a trusted family friend, used by a younger person to address an elder. The word "uncle" had a different meaning in colonial America -- for references see the links in my answer to the question Familial Terminology in Colonial America.
If you are a native speaker of English, and you have to ask someone else what the English relationship term for a particular relationship is, the answer may be that there is no special term. As AdrianB38 says in his comment on the question, your father's step-father is your father's step-father. There may not be any specialized term for the relationship, and even if such a term did exist, you might not wish to use it.
Someone who is close to his father's step-father may simply say "my grandfather", but someone else who doesn't want to be inclusive may say "my grandmother's second husband" or "that person my father's mother married" (or some variant which is even less polite). In a blended family, insisting on being ultra-precise about one's relationship to a person in your family (always calling someone your "half-sister" instead of "my sister", or saying "my stepbrother's son" instead of simply saying "my nephew") might be considered dismissive and rude.
For the standard "cousin / removed" terminology which is commonly found in genealogy, there are many charts and relationship calculators available like the one on Steve Morse's One-Step Web pages referred to in the other answer. The principle is to find the common ancestor between you and your relative, then work your way downward to determine how many levels are involved. The "removed" reference means that you and your cousin are on different levels, that is, not of the same number of generations away from the common ancestor.
The answer to the question, "Are we related -- and if so, what is the relationship called?" might differ greatly between the social sphere and the legal one. If a legal matter of inheritance is involved, it is best to consult a lawyer; for the precise term for a complicated relationship, consult a professional genealogist. For one professional's perspective, see the article by Elizabeth Shown Mills linked to below.