Mobile hotspots (a.k.a. portable hotspot or portable hot spot) are portable devices or features on smartphones that provide wireless Internet access on many devices (your laptop, smartphone, MP3 player, tablet, portable gaming device, etc.).
Like USB modems from wireless carriers, mobile hotspots typically use mobile broadband service from cellular providers for 3G or 4G Internet access. Unlike those mobile USB sticks, though, mobile hotspots allow multiple devices to connect at the same time.
One of the earliest mobile hotspots was the MiFi (Compare Prices), a small credit-card sized device made by Novatel and offered first by Verizon. It broadcasts the 3G cellular signal that can be shared wirelessly by up to 4 devices. (The MiFi was positively reviewed by About's VoIP Guide and chosen here as a top travel accessory). Besides the MiFi, which is also carried on AT&T and Virgin Mobile, there are other similar mobile hotspots, such as Clear's iSpot for Apple iOS devices and 3G/4G Clear Spot.
Besides portable mobile wi-fi hotspots, some smartphones can act as mobile hotspots, sharing their wireless data connection with several devices. The Palm Pre Plus and Pixi Plus had this feature built-in and Verizon offered the hotspot service for free (normally hotspot access garners an additional $10-$20 charge, similar to wireless providers' tethering add-on charges). Verizon introduced a unique 3G mobile hotspot feature with its launch of the iPhone 4.
If you have multiple devices that you use on the go, a mobile wi-fi hotspot can be a critical accessory. Rather than using your cell phone as a modem and connecting it to your laptop with a USB wire or via bluetooth for tethering, you can connect to a mobile hotspot (device or your smartphone) for Internet access anywhere you have a cellular signal. The major downside is that you often need to pay an extra fee for mobile broadband service.