Three main components separate lagers from ales in brewing; yeast, temperature and time.
Ale yeast (saccharomyces cerevisiae) is a top fermenting yeast; it ferments all throughout the wort but rise to the top of the beer towards the end of brewing.
Lager yeast (saccharomyces uvarum) is a bottom fermenter; it ferments all through the wort but settles to the bottom of the beer at the end of brewing.
Ale yeast ferments at warmer temperatures ( around 75F) while lager yeast prefers cooler conditions (46-59F). It is also common to drink lagers colder than that of ales.
Lager yeasts ferment slower because of its cooler environment therefore it takes much more time to produce the final beer.
It can be noted that ales have a lot more flexibility when it comes to experimentation with ingredients but uses much more hops and barley than lagers. As with all processes in brewing there is always the possibility of exceptions to the rule.