Front-end tier -> User Interface layer usually consisting of a mix of HTML, Javascript, CSS, Flash, and various server-side code like ASP.Net, classic ASP, PHP, etc. Think of this as being closest to the user in terms of code.
Middleware, middle-tier -> One tier back, generally referred to as the "plumbing" part of a system. Java and C# are common languages for writing this part that could be viewed as the glue between the UI and the data and can be webservices or WCF components or other SOA components possibly.
Back-end tier -> Databases and other data stores are generally at this level. Oracle, MS-SQL, MySQL, SAP, and various off-the-shelf pieces of software come to mind for this piece of software that is the final processing of the data.
Overlap can exist between any of these as you could have everything poured into one layer like an ASP.Net website that uses the built-in AJAX functionality that generates Javascript while the code behind may contain database commands making the code behind contain both middle and back-end tiers. Alternatively, one could use VBScript to act as all the layers using ADO objects and merging all three tiers into one.
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