Adwords From Google
This is Google's answer to PPC. Instead of listing the results at the top of the listings your ppc results are shown in a long strip of 'sponsored ads' to the right of the search results.
The principle is the same, you state how much you are willing to pay per click to your site and google will place your ad according to your competitors bids. The Adwords results are not however limited to the google search results pages alone. On many web sites today you will see that familiar 'Ads by Gooooooogle' at the top of an ad block.
Using Adwords is a bit of a numbers game. If you know how many warm prospects it takes to sell one of your products, say 2 products sold for every 100 visitors looking for your keyword, then you work out your profit margin on those 2 products and by that you can calculate roughly how much of that margin you can feasibly spend on advertising, divide  it by 100 (the amount of clicks to generate the  2 sales) and that is the maximum amount you can bid for.

An example:
You buy blue suede shoes for $20 and sell them for $50.
You know that out of 100 visitors to your site looking for blue suede shoes that 2 of them will buy.
Your profit margin is $60 for two sales
Divide it by 100 and you get $0.60.
So a bid of $0.60 per click would cost you $60 to make your 2 sales and your $60 profit.
Therefore, as a rule of thumb, half that amount you can put onto you maximum bid...   $0.30 per click maximum
So if you bid for shoes (who's top bid was $0.71) then at that price you would get listed very few times or not at all.
If you bid for 'blue suede shoes' then you might possibly get a number 2 spot (the top bid being $0.34). This would bring you a lot of traffic and you would have to keep forking out $30 for every 100 visitors but with the assurance that you will probably make $60 each time you do this. Depending on how many daily searches are performed for your particular search term, you could be getting (and paying for) thousands of visitors a day giving you a substantial amount of sales.

As I stated in the beginning, Google Adwords is a numbers game. If you like playing numbers game (as I do) then I  highly recommend the book by Greg Helsin entitled Google Adwords 123.
I can wholeheartedly recommend this book (I only ever recommend products I have used personally and had success with). Following the advice in this publication, I started to earn 'real money' within a few weeks.
The formulas within really does work and you are provided with step by step guides to affiliate marketing with Adwords that is literally a foolproof system for generating income.
You do however have to lay out (albeit a small amount) of cash in order to reap the benefits. I can say that the author is to be trusted and can deliver exactly what is promised.(Money back guarantee backs this up) .

Articles
Traffic to your site can be generated by writing specialist articles on your subject, and submitting them to the various online 'Article Directories' for inclusion. Thousands of people regularly scour these directories looking for free content for their newsletters, ezines and web pages. Being as you are allowed to include a link to your site at in the form of a resource box at the end of each article then not only do you establish yourself as an expert in your field but also you instill a confidence in anyone who does visit your site making them a lot more open to buy, its as if they know you already as the person who gives free information without the hard sell.

 
Forums / Newsgroups
Blatant advertising in online forums and discussion groups is generally frowned upon (though some forums offer a space for specifically for this). Traffic may be generated through this medium by making contributions to the forums (in your specialised field) and gaining a 'reputation' as someone who can be trusted, also you are connecting a real person (yourself) with your web site. You are allowed to use a signature in your posts, which normally may include a URL, also you can mention your products or services without actually trying to sell them with a note that if anyone need to they can contact you by PM (private message) for your web address. This way, you are not blatantly advertising but are giving people who are really interested an opportunity to get to what you have to offer at your site. Its a bit of a long winded way to get traffic but the traffic you do get from this method has a MUCH higher conversion rate than with other methods. People are much more inclined to buy from someone they know and if you have been posting on the forums your prospects feel as if they know you.

Classified ads
There are less of these free classifieds sites around today than a few years ago but my advice is to stay away from them. The 'blast your ad to millions' is very old hat now. Really, the only people who visit these free ads sites are those who want to post a free ad..  they are selling , not buying. Your only hope of traffic from these sites is if the very next person to post after you happens to be interested in your product. Very little traffic and generates a LOT of SPAM.

Blogs
A blog (short for web log) is a small dynamic web page that you can easily add text (and images) to. It is like a one post bulletin board that you control, if you wish you can allow comments to be posted by others too.
A lot of people use a blog in conjunction with their site to keep up to date information and news.
The promotion of blogs seems to be in its own category having dedicated blog directories and search engines. There are at present far less blogs than web sites and it is sometimes easier to promote a blog than a web site. Some webmasters are now promoting their blogs and linking heavily to their web sites from with their blogs.

Offline publicity.
Depending on your business this may or may not be a viable form of drumming up visitors to your web site. If you sell a physical product then by all means make sure that everything that leaves your hands has your web site address on it. This is free advertising and people who are really interested in your product WILL visit your site (especially if you have an easy to remember URL).