Top-level domain (or simply TLD) is one of highest level domains in Domain Name System hierarchy on the Internet. TLD is actually the last the last part (last label) of fully qualified domain name. In other words for "html-codes.net", TLD is "net".
Now, there is a whole lot of debate on whether choosing the right TLD has chance to give you better ranking with search engines or not. The facts are a bit complicated, as there is no simple true or false answer. Globally speaking all TLDs have equal ranking as far as crawlers go. It simply does not matter whether you choose .com, .org, .net, .info, or every day more and more popular .co or even any national domain. In general terms, TLD does not influence ranking of your website or a page at all. And this is a raw core fact. Any domain for your website carries the same weight SEO wise.
But, yes there is a but, there is a modifier for local domains. Which means national TLDs will have a slight advantage in local searches. But only a very slight advantage nothing more, they will not rank higher at all. The reason for this is simple. If you conducted a search on google.co.uk, it is only logical that among equally ranked websites the one with .co.uk TLD willl have some advantage over coms, orgs, nets etc. While an English language might not be the best of examples, take for example a search made on google.fr (Google search in French). So if a user from France (in our example) searches for some term, it is only logical that he would mostly appreciate an answer from a French (.fr) website. But like we've said this does not mean that Google (French version) ranks .fr websites more than those with other TLD, it simply means the same information of equal quality available in local language (or only on local TLD) might be more appropriate for the person.
However please note that reverse does not apply. A search made on google.com will not give any, not even the tiny advantage to coms. Which means other parameters (the real ranking modifiers) will have the sole influence here.
Branding and visitors
This factor actually has some influence, but ot on SEO, it influences the visitors themselves. In general terms a lot of people simply search for the information they need via some search engine, or bookmark pages they wish to visit again. But very often people just enter the website address in browser's address bar. The statistics show that majority of people try to shoot for .com, .net, .org or .info in that order when they try to reach some global resource, or aim at national TLD when trying to get to localised website. This is why you should always try to figure out, who it is you are trying to reach. If you aim at only one language or population at certain country, you should choose a national TLD. If you aim at a global public one of the top four popular should be your choice.
Putting it simple, you should rather consider your brand and (aimed) visitors friendly TLD, then forcing a keyword presence in your domain name. Like mentioned, no TLD will influence your SEO ranking, but they will influence how your visitors react to it. For example we would never recommend .co domain, even though it seems more and more popular nowadays, from a simple reason that peoples habit will cause a lot of them to enter .com by mistake.
Purpose and choice
One more thing to consider is your visitors psychology. Which brings us to previous paragraph. What content does your website have? Who is it aimed at? Who is it that you want on it and who you do not want? These are some of the questions you should answer to yourself before the actual TLD choice. Is it a broad audience, or only local, and what population structure? Once you get these figured out, the TLD choice will come easy.