Traditionally, a guitar is played with the neck pointing to the left. This allows the right hand to do the more complex task of picking/strumming. Although playing guitar with a pick has become more popular than finger-picking, the action of strumming is still usually considered more demanding than fretting, so many left-handers prefer to play left-handed guitars (in which the neck points to the right).
Some older musicians found creative ways to play guitar left-handed: Albert King simply flipped his guitar and played upside-down, while Jimi Hendrix used right-handed guitars but re-strung them for left-hand playing. Today, left-handed guitars have become more common and easier to find, but may still be more expensive than right-handed guitars.
A disadvantage of playing left-handed is that you will not be able to pick up and play a standard guitar if one is laying around; you will typically have to plan and bring your own.