The terms 'co-marketing' and 'co-branding' are often used interchangeably, but they can have slightly different meanings. The overall concept is that two companies or brands work together - either for a one-off campaign or as part of a longer term plan - and combine their marketing efforts to achieve greater exposure and more effective results. It's generally a case of identifying symbiotic dynamics and complementing each others' products in some way in order to gain a larger overall market share than they would be able to otherwise achieve on their own, without collaboration.
Although products being marketed as part of a co-marketing campaign may remain unique to each brand, sometimes two companies will create a new product together - essentially fusing their brands - and this will then be sold as a co-branded product.