Google and other search engines will index your webpages, while doing so they take into account words that stand out. This can be in titles, headers and also when text is made bold/strong. Many people say that you should make your main keyword for that webpage bold within the content somewhere to give it that extra little help. Never go making loads of keywords bold on the same page though, you can easily over do it.
Google used to favor the old fashioned <strong> tags over <b>, but for the past 2 or 3 years they have been treated the same.
Bolding your keywords shows a very little emphasis to the search engines and may help boost the factors that help in ranking your web pages. Recent tests have shown that italicizing keywords provides more of a boost than bolding though.
Both give result same but Google gives preference to <strong> , so mainly target your main keyword for bold don't over use it.
Could anyone give any indication as to what would be overusing it? I think I tend to err on the side of caution when it comes to using <strong> or <b> and only actually highlight a couple of words in a 4/500 word page. I'd love to know if anyone has any opinions as to the frequency; obviously bolding every other word wouldn't be the best strategy, but will one word in each paragraph be at the level of penalisation or enhancement? Everything to do with SEO seems to be such a fine line between doing things too much or too little! Heh.
Just like using header tags, it's definitely important not to overbold an article. I think only use bold as much as you would in any normal article (i.e. without thinking about google), which generally isn't too much - I personally think google is set up to find content that has been set-up only for SEO and doesn't seem natural, which would include documents full of bold words and paragraphs!