clustering often refers to putting a bunch of machines together to solve one big computational problem all together, while load-balancing always means using a bunch of machines to parcel out a stream of individual jobs, each handled by only one machine.
Clustering implies that multiple servers can work together, typically for scalability and/or availability purposes. That "working together" implies that they have some knowledge of each other, as opposed to a "farm" of servers that are independent and need no knowledge of each other.
A load balancer takes incoming load and distributes it across a number of servers, for example a farm or a cluster.
Peace,
Cameron Purdy
Tangosol, Inc.
http://www.tangosol.com/coherence.jsp
<a href="http://www.tangosol.com/" target="_blank" rel="nofollow">Tangosol, Inc.</a><br /><a href="http://www.tangosol.com/coherence.jsp" target="_blank" rel="nofollow">Coherence</a>: Easily share live data across a cluster!