Means-plus-function claims, or functional claiming, is a drafting technique. Patent lawyers may use this technique to try to get a very broad patent, but these patents are sometimes easier to invalidate.
The name comes from the US patent law. (TODO: does this technique or this type of claim exist in other patent systems?)
 What is a means-plus-function claim?
The US Code, Title 35 § 112, says:
(f) Element in Claim for a Combination.— An element in a claim for a combination may be expressed as a means or step for performing a specified function without the recital of structure, material, or acts in support thereof, and such claim shall be construed to cover the corresponding structure, material, or acts described in the specification and equivalents thereof.
So a patent can claim a "means for" doing something, but this won't cover all means for doing that thing, it will only cover the specific means which is/are described in the rest of the patent. If the patent doesn't describe any means, then the patent can be found invalid.
 What's this got to do with software patents?
Patent drafters sometimes write software patents with claims to means of doing things, but they don't give the algorithms for exactly how to do that thing. Sometimes these patents can be invalidated in court.
(This section is unfinished.)
 Related pages on en.swpat.org
- Legislation in the USA
- CLS Bank v. Alice (2012, USA) - (I can't remember how relevent this is - I'd have to reread the case)
- Means Plus Function: What Structure are you Referring To?, 10 April 2014, Patently-O
- Another Means-Plus-Function Patent: Invalid as Indefinite, 22 Oct 2013, Patently-O
- The comments to this article are also informative
- Patent_claim_types -> Means-plus-function, Wikipedia
- EFF brief for the case CLS Bank v. Alice (2012, USA)
- The Vast Middle Ground of Hybrid Functional Claim Elements, 20 Oct 2014, Patently-O
- "Another Means-Plus-Function Patent: Invalid as Indefinite". http://www.patentlyo.com/patent/2013/10/another-means-plus-function-patent-invalid-as-indefinite.html. "the courts have also repeatedly held that a patent is invalid as indefinite under Section 112(b) if a claimed "means" if no particular corresponding structure is disclosed in the specification. See, e.g., Function Media, LLC v. Google, Inc., 708 F.3d 1310, 1319 (Fed. Cir. 2013); Blackboard, Inc. v. Desire2Learn Inc., 574 F.3d 1371, 1382-83 (Fed. Cir. 2009)."
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