Call to action is a term you rarely hear outside of marketing circles. In fact, I'd wager it is probably the least overused and/or cliche marketing term of recent years. This is doubly funny because it is both effective as a process and has a wonderful and catchy name that just begs to be randomly peppered in a conversation to prove you know marketing.
A call to action is just that; a word or short phrase that goads the would-be consumer into a transition from passive observer to active customer/client. Great examples of these are: SUBSCRIBE----BUY NOW----FREE TRIAL----JOIN
But the 21st century has seen a rise in short questions that spur consumers.
'Is your website being listed on Google? Enter your URL to find out'
'Claim Your Free Trial'
'Discover Exciting New Recipes'
These are excellent because they engage the curiosity and imagination of people who are already interested.
Another strategy is to give two options, one of which is clearly negatively worded. My favorite of these is Amazons: 'Click here for a free prime trial!' is juxtaposed against 'No thanks, I don't want free shipping.' These are terrific because they make you feel like a rube for NOT spending time or money on their good or service. I genuinely feel bad every time I respond negatively to these, and there have been instances where that step...that call to action, was the nudge I needed to buy whatever they were selling.
A call to action cannot be understated. Use it properly and it will garner customers/clients. Use it poorly and you're fishing with the hook showing.