The Nazca Lines are an extensive, man-made set of 'geoglyphs' (earth signs) created in the Nazca Desert in southern Peru. The majority are in the form of simple straight lines, but there are also depictions of animals and geometric forms etc.
The glyphs are very large - some of the lines are several kilometers long - and were created in the period between 500 BC and AD 500. Despite their age, many are very well preserved due to the lack of rainfall.
Their significance for the Nazca people who built them is unknown - but there are a large number of theories as to their possible purpose(s).
The following are some of the accepted academic theories :
See Wikipedia
Despite many years of extensive research and speculation - their first documented description goes back to 1553 - there is currently no general agreement about why they were built.
Further detailed info with photos Wikipedia
Zoomable / scollable aerial imagery of the lines here