Oblique or Sloped Roman letterforms are sometimes incorrectly called italic. Like italics, obliques are used to offset, or give emphasis to, certain parts of a text otherwise set in upright roman type.
Oblique styles may also be called slanted roman since they are the same letterforms found in the roman face but have been slanted. In some instances they are mechanically slanted (where a slant is created by the font software) or hand-drawn slanted by the designer.
Italic letterforms (sometimes called “true italics” to further distinguish from obliques) have been drawn so that they are more formally differentiated from upright roman letters.
Adrian Frutiger’s Frutiger uses obliques in lieu of Italics. But Frutiger Next (from Linotype) has italics, for example.
There are various schools of thought about whether obliques or italics are more proper for the emphasizing of text. Stanley Morison was briefly in favor of sloping letters instead of true italics. The divide may have begun at that time.
Most desktop publishing and word processing applications will slant the roman on the fly if the italic or oblique style of a given font is not available. It is considered incorrect in professional design and publishing circles to use artificial obliques in this way.
Many sans faces have accompanying obliques rather than italics. For example, Univers, Helvetica, Futura and Gotham. Some modern sans faces are being designed with custom drawn italics, such as Nexus Sans, Trebuchet MS, Myriad, etc.
For more about the visual characteristics of italics, see the wiki entry for Italic.