Social Media Marketing is about developing a strategy to keep your business top of mind for your followers by putting out content that appeals to your audience and encourages engagement such as reviews, comments, likes, and posts to your page, etc. so that your content will show up in your followers feeds and their followers feeds on a regular basis. It’s also about using paid ads on those platforms to reach a broader audience that matches your ideal customer either by demographic, proximity, or interest using content with a call to action appealing enough to make them either come in to your business, purchase online, or use a service that you provide. In a nutshell.
Search Engine Marketing is paid advertising and pay-per-click advertising on search engines like Google, for instance if you pay for a PPC ad, Google will place your business at the top when someone searches for your type of business/service/product with a little ad indicator for as long as you are paying for the ad and then each time someone clicks on that ad they charge you a fee. This is meant to increase visibility with the goal of increasing your chances of being seen and ultimately clicked through to your website at the exact time that a potential client is looking for you or what you offer/provide. (This is different than Search Engine Optimization)
Social Media Monitoring is just like it sounds. You monitor the conversations going on. You monitor trends. You monitor your own reputation. You monitor your competitors, and others in the industry to see what’s working. You will track content, engagement, hashtags, keywords, times of day when your followers tend to be online and non followers that could be potential clients, etc. You monitor basically anything that could potentially help you develop better campaigns on those platforms and increase ROI.