"In physics, a world line of an object (approximated as a point in space, e.g. a particle or observer) is the sequence of spacetime events corresponding to the history of the object. A world line is a special type of curve in spacetime. Below an equivalent definition will be explained: A world line is a time-like curve in spacetime. Each point of a world line is an event that can be labeled with the time and the spatial position of the object at that time."
http://www.worldhistory.com/wiki/w/world-line.htm

Here are a few additional pages containing references and explanations:

http://foko.komputilo.org/~sencay/AT/art/universlinio.html

http://www-lsp.ujf-grenoble.fr/vie_scientifique/gdr/GDROX/au... (see page 9)



--------------------------------------------------
Note added at 33 mins (2004-06-11 20:39:24 GMT)
--------------------------------------------------

And here is a slightly clearer explanation in English:

\"The Curvature of Spacetime
First of all, it is important to remember that spacetime is curved, not merely space. If you want to try to visualize spacetime as a curved surface in 3-space, you must think of it as a universe of one-dimensional beings living in one dimension of space and one dimension of time. Each bit of matter is not a point on whatever curved surface you imagine, but a line showing where that point moves as it goes from the past to the future. These lines are called world lines.\"
http://www.fact-index.com/g/ge/general_relativity.html