The quantity of characteristics which are used to characterize language will vary, depending on the text referenced. Generally speaking, the following are considered common features of all languages.
Language is social. By its very nature, language evolved to connect humans and to enable them to interact with each other. Users of a language agree to a set of rules and systems and then use those rules consistently so that they can better function in their society. Language exists because humans depend on each other and therefore need a reliable way to interact with each other.
Language is arbitrary. The choice of any particular sounds to represent a thing or idea is arbitrary. Why is a feline animal called a cat in English, a gato in Spanish, a mace in Albanian, and a kot in Polish? There isn't any particular reason that certain sounds have been ascribed to the things they represent in a language, but once the word has been established, users of that language agree on the choice.
Language is systematic. Language is put together in a certain way, which varies depending on the language. There are rules for the way letters can combine to form words and the way words combine to form sentences. There is a grammar that establishes the syntax of the language and governs the way tenses are conveyed. This system is fairly predictable (with a few exceptions along the way), and users can thereby manipulate the language to fit their needs based on the rules of the language.
Language is symbolic. There are various symbols and sounds that represent meaningful ideas to users of a language. When speakers of English (and many other languages) see a ?, they understand that this symbol represents a question. Words themselves are symbolic; when readers see the word I on a page, they understand that this is more than a vertical line. These lines represent a first-person pronoun, identifying a speaker. Interpreting symbols correctly is important to the speakers of a language.
Language is dynamic. Any given language wasn't developed in a day, and language continues to evolve over time as speakers and cultures change. Fifty years ago, there was no need to have a word for app, downloading, cyber bullying, smart cars, or online learners. Conversely, it's rare for users of modern English to use the words elflock, lunting, giglet, shinnicked, or primprit, though these words were once commonly used. In America, using some words can date users. Coolio was a popular term in the 1990s. Far out is often associated with the hippie culture of the later 1960s and 1970s. Icebox is often used by an older generation who originally owned the technology requiring a block of ice to keep food cold. Language is dynamic, becoming what a culture needs it to convey.