Before the 1969 reform of the Roman calendar, Saint Christopher was listed as a martyr who died under Decius.  Nothing else is known about him. 

There are several legends about him including the one in which he was crossing a river when a child asked to be carried across.  When Saint Christopher put the child on his shoulders he found the child was unbelievably heavy.  The child, according to the legend, was Christ carrying the weight of the whole world.  This was what made Saint Christopher patron saint of travelers and is invoked against storms, plagues, etcetera.

Before the formal canonization process began in the 15th century, many saints were proclaimed by popular approval.  This was a much faster process but unfortunately many of the saints so named were based on legends, pagan mythology, or even other religions (for example, the story of the Buddha traveled west to Europe and he was "converted" into a Catholic saint)! 

In 1969, the Church took a long look at all the saints on its calendar to see if there was historical evidence that the saint existed and lived a life of holiness.  In taking that long look, the Church discovered that there was little proof that many "saints," including some very popular ones, ever lived.  Christopher was one of the names that was determined to have a basis mostly in legend; therefore, Christopher (and others) were dropped from the universal calendar.

Some saints were considered so legendary that their cult was completely repressed (including Saint Ursula).  Saint Christopher's cult was not suppressed but it is confined to local calendars (those for a diocese, country, or so forth).  His name Christopher, means Christ-bearer.