There are two types of USB devices as far as power management goes: low-power devices and high-power devices. Low-power devices are required to take just a trickle of power. High-power devices are required to start in a low-power mode, then interact with the computer and request a larger power budget. If the computer doesn't have the power, it will reject the request, and the device will not enter high-power mode (and thus won't be accessible).
The power provided on a USB port is limited by the spec. Generally, a USB port can power at most one non-low-power device (and not always even that, depending on how high "high-power" is). You can plug multiple low-power devices in, however.
A passive hub (one that is powered by bus power) can provide power for several low-power devices (some USB keychain drives, most keyboards, most mice, and any device with its own power supply), but it generally cannot provide power for high-power devices (or one at most). Because a powered hub is not constrained by the available bus power, it can power (up to) one high-power device per port on the hub.