The Wide Field Camera 3 (WFC3) project was originally conceived to only replace the capabilities of an aging WFPC2 instrument. During the later phases of study for the instrument however, it became clear with the advancement of technologies and careful planning, WFC3 could substantially enhance Hubble's abilities by adding a second channel (in the near-IR range). Adding a second channel of this type is almost like adding another instrument to Hubble.
With these two channels, WFC3 will achieve excellent panchromatic (full - spectrum) imaging. Stellar objects are not just in the visible spectrum, but also exist in the blue (near-UV) and red (near-IR) extremes. WFC3 was designed to study light in these regions of the spectrum better than Hubble's current capabilities. Specifications of WFC3's channels can be found in the technology section.
Because of advances in detector technology, WFC3's imaging capability will be the best yet. One of the most important specifications for Hubble's instruments is resolution. The better the resolution (smaller value), the more detail can be achieved when imaging stellar objects. It is measured in angle-size per pixel. Below is an example of a pixel with a 0.13 arcsec resolution.
WFC3 is superior to WFPC2 in resolution and field-of-view. It will be comparable to ACS (the most advanced instrument currently aboard Hubble) and excel in some areas. Plus, its IR-channel will be a great enhancement to Hubble's infrared capabilities. Specifications of WFC3's detector capabilities can be found in the technology section.
Side Note: The specification often given with digital cameras is its megapixel Resolution. A 4.0 megapixel camera is common in the marketplace today. WFC3 UVis channel consists of a square array of 4096x4096 pixels yielding 16.8 million pixels or a "16.8 megapixel camera". The IR channel would be a "10.5 megapixel camera".