The lexicon includes the lexemes used to actualize words. Lexemes are formed according to morpho-syntactic rules and express sememes. In this sense, a lexicon organizes the mental vocabulary in a speaker's mind: First, it organizes the vocabulary of a language according to certain principles (for instance, all verbs of motion may be linked in a lexical network) and second, it contains a generative device producing (new) simple and complex words according to certain lexical rules. For example, the suffix '-able' can be added to transitive verbs only, so that we get 'read-able' but not 'cry-able'.
Usually a lexicon is a container for words belonging to the same language. Some exceptions may be encountered for languages that are variants, like for instance Brazilian Portuguese compared to the Portuguese language, where a lot of words are common and where the differences may be marked word by word.
When linguists study the lexicon, they study such things as what words are, how the vocabulary in a language is structured, how people use and store words, how they learn words, the history and evolution of words (i.e. etymology), types of relationships between words as well as how words were created.
An individual's lexicon, lexical knowledge, or lexical concept is that person's knowledge of vocabulary.