Wikipedia: Be

The infinitive form of the copula in the English language. See below.
In chemistry, symbol for Beryllium. Chemical element; atomic number 4 in the periodic table.
An informal name for Be, Incorporated, a software company offering the BeOS operating system for personal computers. The company sold its assets to Palm, Inc in 2001.

The English word be has many meanings, which are the subject of discussions in grammar and ontology. Linguists call the verb "to be" a copula, when it indicates identity, belonging, or the possession of attributes; that one thing is a special case of another, or that it simply is anything. One can distinguish, as logicians do, the possession of attributes from class membership. We can, therefore, identify four senses of "be" and its conjugations.

To exist. "I want only to be, and that is enough." "God is" (a way some theist?s assert their theism). "There's no sense in making a scientific inquiry about what species the Loch Ness Monster is, without first establishing that the Loch Ness Monster indeed is."
Identity. "I only want to be myself." "When the area behind the dam fills, it will be a lake." "The Morning Star is the Evening Star." "Boys will be boys." "I yam what I yam" (Popeye).
Class membership. To belong to a set or class: "She could be married." "Dogs are canines." "Moscow is a large city."
Predication (property and relation attribution): "It hurts to be blue." "Will that house be big enough?" "The hen is next to the rooster." "I am confused." Such attributes may also relate to temporary conditions as well as inherent qualities: "I will be tired after running." "Will you be going to the play tomorrow?" (In this latter form, some grammarians might consider "be going" single verb phrase with "be" as a "helper verb", while others consider the gerund? "going" to be used as an adjective describing a condition parallel to the "tired" example. This latter interpretation is more consistent with other Germanic languages).

In ontology, philosophical discussions of the word "be" and its conjugations takes place over the meaning of the word is, the third person singular form of 'be', and whether the other senses can be reduced to one sense. For example, it is sometimes suggested that the "is" of existence is reducible to the "is" of property attribution or class membership; to be, Aristotle held, is to be something. Of course, the gerund form of "be," being, is its own (vexed) topic: see being and existence.

Other languages have multiple words for the verb "to be", dividing its uses in different ways. For example, the Japanese language has two forms: "arimasu" for the existence, and "desu" for identity and the property-posession uses. The Spanish language also has two words, but divided differently: "ser" for uses expressing permanence (whether existence or attributes) and "estar" for temporary conditions, either of existence or attributes. These are the kinds of issues that make machine translation difficult. For example, the English sentence "I am strong" would be translated into two different Spanish sentences depending on whether the speaker intended to express that this was an inherent quality he posessed ("Soy fuerte"), or a present condition based on circumstances ("Estoy fuerte"). Finally, the divisions are not always clear and consistent. To say that "a book is on the table", for example, Japanese would use the "arimasu" form, saying "Hon wa, taberu ni arimasu," meaning roughly "as for the book, there exists on the table", while Spanish would use the "estar" form "El libro está en la mesa", meaning roughly "the book presently posesses the property of being on the table".

In the Russian language, the verb byt' is the infinitive of "to be." The third person singular, yest' means "is" (and, interestingly enough, it is also the infinitive "to eat") but there is no present- or future-tense copula in the Russian language. Yest' (in the non-gustatory sense) is restricted in use to express existence; On yest' means "He exists," while On yest' krasivyi is nonsense (though it is how an English speaker might try to translate "He is handsome"). There is a past-tense copula in the Russian language. E.g., one can say, Ona byla krasiva: "She was beautiful."