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A gene is a section of DNA within the genome that codes for specific aspect of a living thing. Usually a gene corresponds to one or more proteins that it codes for, by transcribing the gene's DNA information to a string of RNA, which then is fed into a ribosome to produce a protein. A gene may code for more than the one protein due to the same chain of amino acids being folded in more than one way, and because of the ability for genes to be 'spliced' in such a way as to produce entirely different proteins. The gene may also be part of the gene regulation system which controls or modifies how a given gene is expressed, significantly during embryonic development.
Genes are organized into chromosomes more or less randomly, though some genes, for example the HOX developmental genes are organized by order of expression. The randomness of organization means that genes can be linked by being close enough together to avoid being split during meiosis; this results in correlations such as blue eyes and blindness in cats, which are not directly causative.
Genes code for a lot, but not all, of the features seen in life, and usually outside factors such as environment will also have a hand in shaping the final form. These outside influences are studied in epigenetics. Genes were also the inspiration behind memetics, where units of cultural information (as opposed to genetic information) are referred to as memes.