Ankit, you have very nicely explained the difference between SEO and SEM. I understood them to be part of it each other, but never gave it a proper thought. Search Engine Optimization (SEO) is a technique that helps raise up our (search) rank bring more traffic to our website. While Search Engine Marketing (SEM) is a larger concept that includes SEO as well. Internet Ads are an important component of SEM.
Setting up the SEO Writing Assistant is pretty easy and is available at present only for the Google Chrome browser and WordPress. It is compatible with Google Docs which is a good start. To begin with, you need to head over to the download page, log-in (if you don’t have an account one will need to be created), choose to install the browser plugin and activate it post installation.
Brand Monitoring also allows you to get insights into how competitors and other industry leaders are creating opportunities which you can seize. You can research their present and past activities which then means you’re able to adopt their best practices and find ways to distribute your content saving you time and money which would otherwise be spent on trial and error. So, in a way you are learning from what the competition is doing right.  
Important note: The code is already active and will remain active for the next couple of days only. It gives 14 day free access to SEMrush Pro account. You are required to enter the credit card information to sign up for the above trial, but you can cancel it at any time before the trial expires and if you forgot to, within 7 days after you get billed (guaranteed refund time). So you won’t lose anything if you are not satisfied with their features.
Ahrefs isn't cheaper and has a less appealing interface and layout. SEMrush is aesthetically pleasing, but you need to be willing to allocate budget to it and to make use of the comprehensive array of tools it brings to the table to recoup that investment. Free plans might suit if you only want 10 keywords to look at; if not, buckle up. Use it right, and it'll be worth it.
Their SEO content template tool is also a great one for beginners. You have to just put the keyword you want to rank for and the target geo and it will provide you a template to do that. It'll tell you the keywords you need to use in your content and on-page elements and the domains from which you need to rope some backlinks to make your piece rank on the first page.
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