SEO stands for search engine optimisation and refers to the techniques used to increase the visibility of your website in search engines like Google, Bing and Yahoo.
Have you ever been confused by the title of a search result and had to read the bit of text below it to figure out if that result is what you're looking for? That bit of text is called a meta description and if you write a good one, they're a great way to bait a user into clicking on your search result and not somebody else's.
Yes but only to an extent and a keyword domain name does not necessarily fit all industries. At the end of the day, your domain name should be the same as your business name, not ridiculously long and hard to remember, and not stuffed with keywords.
Look for natural opportunities to drop a keyword into your content, where it won't look out of place or irrelevant to the topic of that section of the content. Also keep in mind that your website can and will be penalised by Google if you over stuff your content with keywords.
Alt (alternative) text on an image helps the Google bot that crawls your site figure out what the image is as it cannot "see" an image. So make sure any images on your website are relevant and maybe even drop a keyword directly related to the page the image is on in the alt text.
This depends on a variety of factors, including the age and size of your website. It can take anywhere from one week to one month to see results and be able to make decisions on your SEO strategy.