A transit of Venus occurs when Venus passes right between Earth and the Sun: it looks like a tiny black dot, about 1/30th the size of the Sun (more technically, about 1 arcminute in diameter), slowly going across the Sun. You'll be able to just barely see it with the unaided (but protected) eye, but a far better, magnified image will be visible if you look through binoculars (properly filtered) or a telescope (again, properly filtered). If you wish, you can get very nice, appropriate equipment at many stores, such as Scope City. Wherever you live, local amateur astronomy organizations and science centers will set up telescopes for public viewing, you'll just have to check online for locations. The Chabot Space and Science Centerin the Oakland hills is one such example.
In the Pacific Daylight Time (PDT) zone, the transit starts on June 5 at about 3:07 pm, though it will be hard to notice until around 3:24 pm. Mid-eclipse will be at 6:27 pm. In the San Francisco Bay Area, sunset occurs around 8:28 pm. The transit ends at 9:47 pm PDT, after sunset in California. As with the partial lunar eclipse above, if you are in other time zones, adjust the times accordingly. These times are, of course, approximate to within a few minutes, however they will depend slightly on your exact location within a time zone. So, progressively less of the transit will be visible the farther east you go in the USA.
Suppose you consider the Sun’s disk to be like the face of a clock when you look directly at it. As seen from San Francisco and most of the USA/Canada, Venus will begin to transit at roughly the 12 o’clock position (i.e., near the top of the Sun), but around sunset, Venus will be near the 4 o’clock position. More information on the event can be found at transitofvenus.org, and eclipse.gsfc.nasa.gov/OH/transit12.html.
Transits of Venus are historically important because observations of them in 1761 and 1769 from many different locations on Earth led to the first accurate (to within 2%) estimate of Earth's distance from Venus, and hence of Earth’s distance from the Sun (the “Astronomical Unit”) since the relative distances were already known. Viewed from a given location on Earth, the exact position of Venus’s silhouette depends on Venus’s distance, so one can solve for the distance using a bunch of measurements. Transits of Venus are important now because they illustrate the technique with which thousands of exoplanets are being found by the Kepler spacecraft: the total brightness of a star periodically drops a tiny bit while an exoplanet transits across its disk, as seen by us. If you monitor a star’s brightness and notice a slight periodic dimming, you’ve probably detected an exoplanet.
If you are planning on looking directly at the Sun, remember that you need proper eye protection or you could permanently damage your eyes! Use "Shade 14 welder's glass" (available at welding supply stores) or some other safe filter which blocks 99.999% of the Sun's rays at visible, ultraviolet, and infrared wavelengths. Appropriate "eclipse glasses" can also be found and purchased online for just a few dollars (comparable to Shade 14 welder's glass, but less durable). Regular sunglasses or smoked glass won't suffice by a long shot.
Without magnification, the black silhouette of Venus will be barely visible to the naked eye, or possibly even invisible, depending on your visual acuity. A magnified view will be much better. If you have a sufficiently small pair of binoculars, you can securely tape a piece of Shade 14 glass across the front end (so that light goes through the filter before entering the binoculars). Many local amateur astronomy clubs and science centers will have properly filtered telescopes for public viewing, providing a magnified image. I encourage you to visit them and watch the transit safely.