"Tier" is perhaps a better parlance than "layer". In the
J2EE world, we always speak of three prominent tiers viz., Client Tier(aka Presentation tier), Application Tier( aka Middle Tier) and the Database Tier(aka Persistence tier ). Some architectures include an Enterprise Information Systems(EIS) tier in addition to/instead of Database Tier. The entire set of component technologies offered by the J2EE platform can be divided into these four tiers.
The term layer is often used in the context of design
patterns, to distinguish the boundaries between collaborating components. Pesonally I prefer using the term "tier".
Cheers,