Here is one breakdown:
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.
Similarly, taking middleware and either front or back-end can be combined in some cases.
Bottlenecks generally have a few different levels to them:
1) Database or back-end processing -> This can vary from payroll or sales or other tasks where the throughput to the database is bogging things down.
2) Middleware bottlenecks -> This would be where some web service may be hitting capacity but the front and back ends have bandwidth to handle more traffic. Alternatively, there may be some server that is part of a system that isn't quite the UI part or the raw data that can be a bottleneck using something like Biztalk or MSMQ.
3) Front-end bottlenecks -> This could client or server-side issues. For example, if you took a low-end PC and had it load a web page that consisted of a lot of data being downloaded, the client could be where the bottleneck is. Similarly, the server could be queuing up requests if it is getting hammered with requests like what Amazon.com or other high-traffic websites may get at times.
Some of this is subject to interpretation, so it isn't perfect by any means and YMMV.
EDIT: Something to consider is that some systems can have multiple front-ends or back-ends. For example, a content management system will likely have a way for site visitors to view the content that is a front-end but what about how content editors are able to change the data on the site? The ability to pull up this data could be seen as front-end since it is a UI component or it could be seen as a back-end since it is used by internal users rather than the general public viewing the site. Thus, there is something to be said for context here.
