The seed stage of any organization is clearly embryonic and is therefore more speculative than other rounds of investment. It is there to establish the startup as a going concern, in many cases going as far as to bring a product to market. A seed investment should aim to achieve one of the following:

a)Product Identification b)Marketplace Orientation c)Demographic Targeting d)Team Creation

2. Series A Investment

This type of investment is often the first encountered when the seed stage does not require outside funding. At this juncture most startups have a strong defined idea of what the central goal is. Series A investments should achieve one of the following:

a)Distribution b)New Markets

3. Series B Investment

By the time Series B investment is being actively pursued a startup is usually well on its way to being a truly established business. Production is well managed, advertising in in full flow, and customers or users are actively purchasing an associated product or service as planned. While scalability is a factor in Series A investment, here it is the main focus. This includes:

a)Team Expansion b)Globalization c)Acquisitions

4. Series C Investment and Beyond

There is no technical limit to the number of investment rounds a startup can pursue. This depends heavily on any anti-dilution agreements previous investors have acquired, ensuring that their stake is never watered down. As each investment round progresses more and more equity from the company is released, so they are normally not entered into lightly from both investor and founder perspectives.

Full article here http://blog.onevest.com/blog/2015/4/23/startup-investment-101-investment-rounds-explained