Series A
Series A funding usually comes from a smaller number of VCs and Angels who will invest $2-10 million, in most cases for equity. The series is named after the type of shares the participating investors will (hope to eventually) receive – Series A Preferred. This is usually the first round of preferred shares.
Series B
While in a Series A the goal is usually to support a business model that works, scales and prove that the company can reach well defined goals, the Series B is there to pour fuel on that initial fire. The companies that access a Series B are usually already starting to turn a profit and their major challenge is scaling fast on the cash they have now. At the same time, a company in Series B may be targeting international expansion, or branching out into related product ranges.