Common and paired gender

As you know, there are three main categories of gender in Russian: feminine, masculine and neuter. The endings of feminine gender are -А [a], -Я [ya]; neuter - -O [o], -E [e]; words of masculine gender most often end with a consonant. However, there are certainly some exceptions.

In addition to these categories one can distinguish two more word classes which can be referred to common and paired genders (plurale tantum). Let's look at them.

We refer to common gender words which we use as masculine as feminine genders, so they do not change their form depending on gender. For example names of professions. Most often such words have masculine gender but we use them for feminine one, too.

Врач [vrach] - a physician, доктор [doktar] - a doctor, продавец [pradav'ets] - a salesman, библиотекарь [bibliat'ekar'] - a librarian, кондуктор [kanduktar] - a conductor.

And also the words describing some features, character traits or categories of people. Most often such words have feminine gender but we use them for masculine one, too.

Неряха [niryakha] - a sloven, ябеда [yabida] - a slander, пьяница [p'yanitsa] - a drunkard, умница [umnitsa] - a clever person, сирота [sirata] - an orphan.

We decline these words according to their grammatical gender.
We can say "хороший врач" [kharoshiy vrach] (a good doctor) referring to a woman as well as to a man.
Declension of the adjectives which we use in combination with this word most often depends on a subject's gender.

Он - круглый сирота, она - круглая сирота - He is a full orphan, she is a full orphan.
Он - горький пьяница, она - горькая пьяница - He is a fuddle-cap, she is a fuddle-cap.

However, it's not a strict rule so you can say about a boy - "круглый сирота" [kruglyy sirata].

There are also the words which we use only in plural and decline accordingly. For example:

Часы [chasy], духи [dukhi], ножницы [nozhnitsy], обои [aboi] - watch, perfume, scissors, wall-paper.
Брюки [bryuki], джинсы [dzhinsy] - trousers, jeans.
Шашки [shashki], шахматы [shakhmaty] - draughts, chess.
Близнецы [bliznitsy] - twins.

Last word - близнецы - can be refereed as to paired gender (if we are talking about sign of the zodiac) as to masculine gender, plural (if we are talking about relatives).