In linguistics, grammatical genders, also called noun classes (just as Bender said above), are classes of nouns reflected in the behavior of associated words; every noun must belong to one of the classes and there should be very few which belong to several classes at once. (Hockett, 1958)
It must be pointed that gender in the grammatical sense has nothing to do with biological sex or with what sociolinguists call "gender", as the social role associated with each sex.
Only in some gender-languages, gender exhibits any correlation at all
with sex and in most of these, the correlation is very weak.
In Portuguese, for example, most nouns denoting females receive feminine gender, but not all: what is the gender of "contralto"? And some feminine nouns do not denote females, such "sentinela".
Grammatical gender is distinguished from natural gender by the fact that grammatical gender requires agreement between nouns and the modifiers (demonstratives, articles, adjectives) and sometimes even verbs, used in a sentence, whereas natural gender does not.
Although Bantu languages have gender, for historical reasons, the linguistis who studied these languages have preferred to talk about "noun classes"' rather than about "gender", but gender is there somehow!
The indo-european languages usually have masculine, feminine and neuter genders. Latin had the three, but in some modern descents of it, such as Portuguese, the neuter gender disappeared. In common nouns, grammatical gender is usually only peripherally related to actual gender
Czech, for instance devides the masculine gender into animate and inanimate groups, this sort of division is not found in Italian, for instance. Some linguistis further claim that the Nostratic Language (the ancestor of Indo-european) had human, animal and object genders separated!
There are languages that show no gender marking on nouns. These languages can be divided into two groups:
a) languages that still distinguishe gender, but the distinction is made on modifiers (adjectives, for example), pronouns, and perhaps even verbs --> but not on the noun.
* German would fall into this category, since most nouns give no clue as to their gender other than the forms of the article, determiner, and adjectives they use.
b) languages which have no concept of grammatical gender --> the forms of modifiers used with the nouns, and of verbs, do not change according to gender.
* English would fall into this category. The word "man" is naturally masculine, and the word girl naturally feminine, but the form of the adjective "tall" used with both is still "tall".
Even if English has no concept of gender in nouns, the personal pronouns often have different forms based on the natural gender of the reference, but this is not the same concept.
Gendered pronouns vary considerably across languages --> there are languages that have different pronouns in the third person only to differentiate between humans and inanimate objects, like Hungarian and Finnish. Even this distinction is commonly waived in spoken Finnish. Other languages, such as Japanese, have a wide range of personal pronouns to describe how they relate to the speaker.
That all Indo-European languages evolved from a common ancestor is indisputable. It is plausible to assume that this ancestor language employed a gender system, possibly one with a semantic basis. But what could have caused its modern descendants to assign genders such as masculine and feminine to inanimate objects? And how can a “pure” system evolve into the modern chaos and disagreement? The answer some authors have given to these questions is that the origin of gender is purely formal: some suffixes of sex-differentiable nouns acted as attractors, and created the genders in a purely formal, non-semantic way (Brugmann, 1899). This leaves open the question of what caused sex-differentiable nouns, rather than any other category, to become attractors.
Another possible answer is that in some languages the initially semantic neuter gender was lost, and the void was filled by masculine and feminine genders being assigned to previously neuter nouns. Such a process can be observed today in Russian, where neuter nouns are only 13% of the total, and loanwords entering the language go primarily to the masculine gender, but also to the feminine (Corbett, 1991:317). This hypothesis does not take into account languages that retain the neuter gender, and still assign masculine and feminine genders to inanimate objects (German, Greek, etc.)
An alternative hypothesis is that masculine and feminine assignments to inanimate objects existed even in the original Indo-European ancestor. Although such assignments seem nonsensical today, they might have
“made sense” in the remote past, at least among the few speakers of the ancestor language, based on animistic conceptions of the world. It could have appeared natural to a particular culture that, for example, a stone
is of female sex. However, as the original language evolved, ideas about the stone’s sex changed, too. Since there is no objective way to agree on something like the sex of a stone, the “opinions” among descendant
languages evolved differently. What we observe today appears as a purely formal and arbitrary assignment, since the original “reasons” have been lost. One prediction of this hypothesis is that gender evolution in
such languages should be traceable through a weak agreement between phylogenetically proximal languages.
source: Evolution of Gender in Indo-European Languages - Harry E. Foundalis
An interesting book I've seen in the library is: Greville G. Corbett, Gender, Cambridge University Press.
"It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge."
~~Enrico Fermi (1901 - 1954)~~