There are two types of hard drive in use today with the difference being in their interface. One is the standard IDE parallel ATA (PATA) interface. The other is the newer serial ATA (SATA) interface. SATA hard drives are theoretically faster than PATA hard drives since SATA hard drives have a maximum bandwidth of 150 MB/s and PATA hard drives have a maximum bandwidth of 133 MB/s (ATA133). 'S' refers to serial and 'P' refers to parallel... therefore the data is transferrerd in a serial and parallel mode respectively..
In addition to what's been said, Parallel ATA can address up to 2 devices per channel, hence the parallel nature, while Serial, by design, only addresses one device per channel. As far as PCIe x1 devices, it's the replacement for your average PCI slot in a general purpose nature. The transitioning over is slow going, but there's low-end video cards, TV tuners, and sound cards, for starters. PCI isn't gone yet, so I don't think we'll see much x1 devices in great number and variety for at least a couple more years.
