Two boxers were trying to decide which of them was the best fighter. They decide the obvious way to decide this is with an actual boxing match, so they arrange a date. However, one of the boxers, secretly knowing his opponent is stronger, sabotaged the promotional material. The audience of the fight were told to turn up 3 hours before the real fight was due to happen. The cheeky boxer then constructed a man out of straw, and when the fight came along, he beat it to a pulp. 3 hours later, his real opponent arrived to find he had been declared the loser.
Apply this parable to logic, and you have a picture of the straw man fallacy. Much like in the parable, the straw man fallacy is a deceptive technique in which one misrepresents opposing views or arguments, then proceeds to attack the misrepresentation instead. The real view or argument then appears to have been refuted, taking on the reputation of the misrepresentation.
A common example is the way proponents of Conditional Security often argue against Eternal Security. Eternal Security, also known as Perseverance of the Saints, or simply "Once Saved Always Saved", is the belief that salvation is irrevocable. Once one has received salvation, one cannot lose it. This is not only a clear promise in Scripture, but is also a logical implication of how one obtains salvation in the first place. If salvation is by grace through faith, and specifically not of works, then the only way to lose salvation would be to lose faith, which the Bible tells us proves you never had salvation to lose.
Conditional Security, by contrast, suggests that keeping one's salvation is conditional (hence the name) upon one thing or another. This varies wildly, with some people believing it is conditional only upon "enduring to the end" (i.e. keeping your faith until the day you die), but others believing some sin or other may cost salvation.
The latter kind are very likely to point out that Eternal Security, theoretically, means one can act as horrendously as if one was not saved, yet still remain saved. This, however, is a straw man, simply because it focuses only on one aspect of Eternal Security while ignoring the nuance. Martin Luther said it best: "Salvation is by faith alone, but faith will never be alone". In other words, salvation leads to works, rather than works leading to salvation. Therefore, while one could theoretically sin and retain salvation (and indeed, 1 John 1:8-10 says anyone who claims to be without sin is self deceived and calls God a liar, thus meaning everyone who retains salvation does so in spite of sin), but salvation naturally leads to specifically not acting like that. Furthermore, those who do act like that can be reasonably assumed to be unsaved, not because they lost salvation, but because their fruits suggest they never had salvation to lose.
There is an extra layer to the straw man argument in that one can misrepresent someone by accurately representing another. Denominations, for example, can be described as living straw men, because they genuinely believe what the dissenter is disagreeing with. It is, for example, a misrepresentation to say "The Christian-Jewish pestilence is surely approaching its end now. It is simply dreadful, that a religion has even been possible, that literally eats its God in Holy Communion." While Christianity does have a ritual in which we each bread and drink wine, representing Christ, we do not literally eat Jesus. Nevertheless, the Catholic Church and its offshoots do claim that the bread and wine literally become the real, historical body of Jesus. This particular argument is flawed even against Catholicism, as it comments on how disgusting Catholicism is rather than how true, but as an argument against Christianity, it fails simply because Christianity does not teach that we literally eat our God in Holy Communion.
Typically, the straw man argument is, or at least appears to be, intentionally dishonest, but the absolute best lies have always been, and will always be, those which are closest to the truth. Therefore, the hardest straw man arguments to spot are those which give the best representation of the original argument or view, which of course is likely to happen if one misunderstands one's opponents. Therefore, it cannot always be assumed someone making a straw man argument is doing so deliberately, and the default assumption should, unless otherwise indicated, be that they are honest mistakes. Nevertheless, a mistake, however honest, remains a mistake, and the straw man fallacy can be called out whenever it occurs.