Criticising a religion, (Islam), is not Islamophobia. Prejudice, hatred, or bigotry directed against Muslims in general, is. It's pretty simple.
There is more to it than that, though. Making misleading statements about Islam the religion is also very, very common for Islamophobes. Inflammatory, offensive and incorrect statements are used; these statements have the effect of making any civil discussion impossible.
I would say the definition should also include prejudice, hatred or bigotry directed against Muslims or the religion of Islam.
It's usually easy to tell the difference between someone making legitimate criticisms of religion, and someone who is just a raging bigot motivated by hatred. Islamophobes are immune to all evidence and cannot be reasoned with. They're not interested in a discussion; they've already made up their mind and no amount of contrary evidence will have any impact on them.
One of the most common behaviours of Islamophobes is to conflate Wahhabism with Islam in general, and to deny the existence or validity of any other form of Islam. Another thing they do is present misleading, inaccurate quotations from Islamic scriptures or authority sources, taken out of context, and then attempt to use these as "evidence" for their views - once again, attempting to point out the context of the text, or providing evidence that contradicts their claims, is pointless - these people are immune to all evidence. They are not acting in good faith, as part of a legitimate debate, their only interest is point-scoring and once again, they will dismiss, ignore, or attempt to discredit all evidence or sources that disagree with what they are saying.
Another tactic that they use is to point to majority Muslim societies in the world today, and then use that as evidence to condemn Islam the religion and Muslims in general as being backward, violent, etc. etc. etc. Once again, here they are not being honest. You cannot take countries that are beset by poverty, unemployment, lack of education, constant war, and goodness knows how many other problems, and compare them with countries that are centuries ahead in terms of development. That isn't a level playing field, and it isn't fair.
Yet another logical fallacy that they use is to attempt to apply modern morality (by which I really mean, the morality which has become fashionable in western countries since as recently as the 1990s), and then attempt to judge people who lived in the freaking 7th century AD by that standard. Somehow, they conveniently neglect to do this when those same people idealise the ancient Greeks and Romans (who practised slavery on a vast scale), yet when it comes to Islamic history suddenly they developed a moral conscience? Yeah, right. I call BS. Once again, they are not acting in good faith; instead they are simply out to screw over Islam and Muslims in a dishonest and mean-spirited way, without any pretence of being fair or logical and consistent about it.
Other tactics used by Islamophobes include sexual slander against the Prophet Muhammad, his wives, and so on. They also seek to depict Islam as hostile towards women, even though once again, this argument falls apart under the slightest scrutiny. These arguments are based on medieval, orientalist prejudice, and once again they do not conform to confirmed facts and are largely a product of fiction. Evidence that Islam actually improved the situation of women at the time is ignored, and context (such as the fact that Muslim women had the right to initiate divorce, whereas in England divorce was actually illegal until the mid 19th century!) is likewise ignored and overlooked. Yet again, the efforts of Islamophobes aren't based in reality or an objective, fair look at the facts - instead they're just trash-talking as if the Taliban was the only representation of Islamic society (probably because that's the only information about Islam that they've ever encountered on this subject).
Once again, they demonstrate their remarkable ignorance on the subject. You can tell you're talking to an Islamophobe if they refuse to listen to all counter-examples or contrary evidence, and basically do the equivalent of sticking their fingers in their ears and shouting "La la la la I'm not listening, nah nah". And then continue running their mouth with hateful nonsense that says a lot more about them than it does about the actual subject.
Anyway I'm in danger of writing so much text that my own post could be called a rant. This was unplanned but once I started, it turned out I had a lot to say. Well that's enough from me.