Seamless gutters are a relatively new technology when compared to traditional options. The primary difference is that seamless gutters, as the name indicates, aren't seamed together piece by piece like conventional options. Instead, these gutters are usually custom-made to your home and come in longer pieces that provide more strength and durability. The only actual seams in this type of gutter are those at corners where the system has to bend.
While seamless gutters are generally more expensive and require more installation time, the benefits significantly outweigh the downsides.