B2B means business to business, so if you are selling your products and/or services to a business, that categorizes you as a B2B business.
A few examples:
Consulting Agencies who offer strategic, business advice on their client's operations and other business components
Manufacturers who sell to retailers, wholesalers and are not in touch directly with the end consumer.
Accessories designers who sell to retailers, wholesalers and are not in touch directly with the end consumer.
Notice that a B2B business only sells to other businesses and not end consumers. Their promotional tactics are also different from B2C (Business to Consumer) tactics and below we note some ways businesses promote to other businesses:
Trade promotions / discounts: manufacturers often provide trade discounts to their retailers so they can have higher margins. For example, a canned food manufacturer can offer Walmart an extra 15-20% trade discount to give them a higher margin in return for a better placement on the shelves.
Offering supporting marketing collateral: A lot of consumer packaged goods such as toothpaste, batteries, etc. offer designed product stands that retailers can put in the aisles to help promote the product. Being in B2B it's YOUR job to make sure you can help the retailer move your product so they can make money and in return buy more upcoming product from you.
Bundled Services: B2B services online usually offer business packages for heavy users of their service, making it more cost efficient and getting more users on their platform.
Those are just a few of the ways B2B businesses market to each other. The key tip to keep in mind is that if you are selling to other businesses, make it easy for them to sell YOU, help them make money, and they will keep buying from you.