A browser-based wallet or wallet service is an online account with an external provider where bitcoins can be stored. Examples include accounts on currency exchange Markets, online Services and with ecommerce transaction processors.
An address is a Bitcoin public key to which transactions can be sent. This concept is present in the Bitcoin protocol itself. A wallet is a collection of private keys that correspond to addresses. A private key is necessary to spend from an address.
A Bitcoin address is a single-use token. Like e-mail addresses, you can send bitcoins to a person by sending bitcoins to one of their addresses. However, unlike e-mail addresses, people have many different Bitcoin addresses and a unique address should be used for each transaction.