Your business has a number of channels it can use to reach customers. When those channels are run independently, with separate metrics and benchmarks, that's called 'multichannel marketing'. One campaign may use several channels managed by different teams, but each department - SEO, your physical store, and your social media management team, etc. - is essentially siloed from the others.
Omni-channel marketing, on the other hand, focuses on the full customer experience, and treats each channel like a member of one broader marketing team. The channels seamlessly interact, and the customer - instead of feeling like they have one relationship with the social media team, one with the store’s reps, and one with the website - has a holistic view of the company and its marketing efforts.
This is the goal for most marketing departments. In the infographic below, the team from Salesforce share more about omni-channel marketing, as well as how to transition to it in your own marketing department.