Swaps are a cornerstone of any exchange, and Balancer has a few types for different purposes.
You'll want to use Batch Swaps when you're making a trade that hops through multiple pools. These are useful for swapping between two tokens that aren't in the same pool, and for routes with better prices than naive single swaps.
There is a specific case of Batch Swap called a Flash Swap that enables trades with no input tokens. These are useful for doing arbitrage among Balancer pools. To make a Flash Swap, create a Batch Swap with all your token limits set to zero.
You'll want to use Single Swaps when you're making a trade between two tokens in one pool. While it's possible to do this with a one-step Batch Swap, using a Single Swap will save ~6,000 gas.
In most cases, you'll want to use Batch Swaps. The only time you would want to use a Single Swap is when you're making a trade between just two tokens in one pool. Executing multiple Single Swaps in a single transaction is inefficient and should instead be batched.
Sample gas costs for trades executed through multiple weighted pools
Sample gas costs for trades executed through Weighted->Stable->Weighted pools