Thunderbolt from Intel is the child of PCI Express and DisplayPort set to compete against USB 3.0. It is faster than USB 3.0. It replaces DVI, HDMI, eSATA, and almost everything else. The why, when, where, and how you will use Thunderbolt. (Thunderbolt is a trademark of Intel in USA and perhaps elsewhere.)
Think DisplayPort. That old technology announced by VESA in 2006 for connecting monitors to computers and transmitting the sound in the same cable instead of a separate USB cable. AMD and ATI released the first graphics cards using DisplayPort. Dell released the first monitor using DisplayPort. Eventually Apple jumped on the DisplayPort bandwagon but using a different incompatible connector forcing you to buy Apple monitors for Apple computers. Later, in 2009, Apple made their proprietary connector free and VESA released the connector as the Mini DisplayPort in DisplayPort 1.2.
HDMI is similar to DisplayPort but without the flexibility. HDMI is strictly for video while DisplayPort can carry any sort of data. HDMI jumped ahead of DisplayPort because video producers love HDMI and force it on hardware producers. Video producers love to restrict the ways you can view the videos you buy. DisplayPort lets people add some restrictions in software but HDMI has it built into the hardware.
DVI uses similar wiring and signals spread out over many more wires because each wire carries less data. You have to use dual channel DVI to get the same speed as HDMI. HDMI and DisplayPort can carry the same video with DisplayPort carrying a lot of other data at the same time.
USB uses similar wiring and signals to DisplayPort, HDMI, and DVI. The data is carried on serial loops with several loops operating in parallel. Several loops gives you lots of transmission speed. Increasing quality chips and wiring lets you run the loops faster. USB 1.0 was 1.5 Megabits per second, USB 1.1 was 12 Mbps, USB 2.0 is 480 Mbps, and USB 3.0 is 4800 Mbps. USB 1.0, 1.1, and 2.0 use two loops. USB 3.0 uses 4 loops. The rest of the speed gain is through better signalling between chips.
Thunderbolt from Intel replaces all of them with 10 Gigabits per second (10000 Mbps). The speed is way to fast for anything we use now and will initially be used to connect two servers or a server to a mass storage unit.
Thunderbolt from Intel also replaces SATA at the PCI Express level. SATA 2.0 transfers data at 3 Gbps and the new SATA 3.0 doubles SATA speed to 6 Gbps. Thunderbolt will give you almost double the speed of SATA 3.0. No magnetic disks are as fast as SATA 2.0 and only a few solid state disks are as fast as SATA 2.0. You need several disks in a RAID array on a very fast storage controller to flood SATA 3.0. Thunderbolt will be harder to use at full capacity.
Why use something that is too fast for current hardware? You buy a computer and use it for several years. During that time, storage devices increase in speed. You want something to cope with the new inventions for the next few years. If you are buying a server now or building a very fast workstation for video editing, you buy a computer with HDMI, DisplayPort, eSATA 3.0, and USB 3.0. You do not know how many of each type you will need so you buy a computer with a lot of spare connectors of each type Thunderbolt would replace all four types of connectors with one standard one, reducing the number of unused connectors, the size of the computer, the cost, and give you a slight decrease in electricity use.
Thunderbolt will not be in your next computer. It will be in the one after. You might have one token Thunderbolt connection in your next computer but the computer will have all the other connections for compatibility. The one after that will have several Thunderbolt connections and drop DVI, sSATA, and perhaps HDMI.
Thunderbolt does not save you time or money when you replace one connection with Thunderbolt. Thunderbolt starts saving time and money when you replace multiple connections with a single Thunderbolt connection. You might currently use several external storage devices all connected direct to your computer via long messy cables. The future connection might be one long cable to the first external storage device then a couple of quick short connections from that device to the other devices.
The important thing is to start somewhere buying devices with Thunderbolt when those devices are competitively priced. It is not worth replacing a USB 3.0 or eSATA 3.0 device to get extra speed. It is worth looking for Thunderbolt when replacing older USB 2.0, eSATA 1.0, and single channel DVI devices.
Handheld computers have a USB connection for charging and will focus on wireless connections for most peripherals. Tablet computers have a USB connection for charging, a keyboard and external storage. Tablet computers are too slow to use Thunderbolt. The faster notebooks and desktops can use Thunderbolt for video editing. Servers with external storage will use Thunderbolt. A Web server with a separate database server will use Thunderbolt.
Replacing DisplayPort and HDMI will happen first. Apple jumped on Thunderbolt as a way to cash in on the money Intel will spend promoting Thunderbolt. Monitor manufacturers will convert quickly to cash in on the Apple market and everyone else will follow.
Companies selling external storage devices will be leaping over Thunderbolt to replace the many different incompatible ways of connecting external storage to computers.
Thunderbolt allows relaying of connections. Think about all those notebooks with docking stations to make all those different connections. Thunderbolt could replace the typical docking station with enough extra speed to cover high speed backup and synchronisation to external storage. You could arrive at the office, plug your little notebook or tablet or iPad into a decent size monitor using one Thunderbolt cable. The back of the monitor could have another Thunderbolt cable already connected to an external mass storage device. Your docking station is replaced by one Thunderbolt cable and the power cable.