What is ExpressCard/34? Is it compatible with PC Cards?

The ExpressCard/34 expansion slot is 34 mm wide and 75 mm long, and is not backwards compatible with Type I or II PC Cards, which are 54 mm wide and 85.6 mm long.

According to ExpressCard.org, ExpressCard/34 is:

A new standard. . . [that] promises to deliver thinner, faster and lighter modular expansion to desktop and notebook computer users. Consumers will be able to add hardware capabilities such as memory, wired and wireless communications cards and security devices by simply inserting these modules into their systems. All ExpressCard slots will accommodate modules designed to use either USB 2.0, or the emerging PCI-Express standards.

The same website provides an informative Q&A as well as an in-depth summary of the standard that may be of interest.

What is the difference between ExpressCard/34 and ExpressCard/54? Speed?

ExpressCard.org provides a Q&A with the difference between ExpressCard/34 and ExpressCard/54:

There are two sizes of ExpressCard module. One is 34 mm wide (ExpressCard/34) and the other is 54 mm (ExpressCard/54). Both modules are 75 mm long and 5 mm high. Host systems can provide any combination of slots for either width of module. It should be noted that the 34 mm module has the advantage that it will also work in the slot designed for the 54 mm wide modules but not vice versa.

To summarize, the difference between ExpressCard/34 and ExpresCard/54 is width, not speed. The MacBook Pro has an ExpressCard/34 expansion slot that only is compatible with ExpressCard/34 cards, and not the wider ExpressCard/54 standard.

ExpressCard.org also has photographs of the cards for your reference.