So first things first, what are joint ventures or JV's as they are commonly called. Simply put they are partnerships between two or more businesses or people with the sole purpose of increasing their reach and hence profits by promoting or selling each others products. There are a lot of different types of jv partnerships and are normally determined by what each partner has to offer and also what they are looking for.
The majority of JV partnerships, particularly on the internet, are actually just mutually beneficial affiliate partnerships, perhaps on a larger or more varied scale, but affiliate never the less. The way this works is you might have a product you wish to sell to a larger audience and you find someone who might have a website, or maybe a forum but more often a list in the same niche as your product.
Then you contact that person and suggest a partnership where he/she would promote your product to their list or using a website or blog and you would offer them a commission for every sale they make. What takes this to a different level is and probably causes it to be called a JV instead of affiliate relationship is that you as a product owner are actively searching for someone to promote for you rather than just having a affiliate program and waiting for them to hopefully find it. You would normally offer a higher percentage commission for this as it is more targeted and you know you should get good results.
The most common type of "real" joint venture is simply the exchange of ads or promotions. The easiest and most common form of this is you find someone in a similar niche but not with a directly competing product and then you would send their product info to your list and they would in turn send your product to theirs. No commissions involved just cross promotion.
The obvious advantage of joint ventures is it gets your product or service out to a much larger audience which will in turn obviously result in higher profits. There are certain ways and methods of finding these jv partners and more importantly how to approach them to actually get them to agree to a joint venture. There are a lot of guides and course out there to teach you this but they are ridiculously over priced. It really is not rocket science once you know how so a thousand bucks to learn this is way over the top. I would recommend you check out Liz Tomey's guide to Joint Ventures you can find the link in my signature below as it is a very reasonable price.