A transaction is a signed section of data that is broadcast to the network and collected into blocks. It typically references previous transactions and dedicates a certain number of bitcoins from them to one or more new bitcoin addresses. It is not encrypted, so it is possible to browse and view any transaction to ever be collected into a block.
A block chain browser is a site where every transaction included within the block chain can be viewed. This is useful for seeing the technical details of transaction in action, and for verifying payments.