Obviously, the vehicles used have differences--"streetcars" being smaller lighter vehicles which are suitable for mixed-traffic operation, and have a weight profile similar to busses and trucks. The primary performance drawbacks of streetcar-class vehicles is a smaller passenger capacity, and a relatively low top speed, compared to larger vehicles. Light-rail vehicles aren't really light at all--they are frequently as heavy as other passenger railcars (though not as much as locomotives and fully-loaded freight cars), and require stronger roadbeds and such--the primary difference between a LRT and a "metro" car is the platform height, and LRT almost exclusively uses overhead catenary for power, whereas many metros use third-rail power.
But Jarrett's point is--to the transit patron, the engineering particulars of the vehicle matter very little--what is important is the service profile--which has much more to do with the fixed infrastructure and the stop spacing, then it does with the vehicle itself.
And the term "light rail", which is a useful term when referring to the class of vehicles described above, is found in many different service profiles. It's rare to see LRT running in mixed traffic for any considerable distance (MAX and cars used to share the center lanes of the Steel Bridge; but nowadays cars are limited to the outer lanes only)--but MAX has several different service profiles within the same system.
(Much of the same applies to the term "bus rapid transit", but even moreso--the term gets used for everything from mixed-traffic lines like LA's Metro Rapid, to the extensive busway networks in Brisbane).
Here's a taxonomy which I like to peddle, and how it applies to both rail and bus:
- Level 0: Mixed traffic running, very frequent stops (<1000ft) , sometimes with limited signal pre-emption, queue jump lanes, or similar. Streetcar/tram, local bus. Most of Portland Streetcar counts, as do the vast majority of TriMet's bus lines.
- Level 1: Dedicated lanes, frequent stops (1000-2000ft), usually adjacent to cars, sometimes with limited signal pre-emption, lots of cross traffic. "Tramway" seems to be a common name for this in Europe when referring to rail; "bus lane" is a common term for this in the US for busses. The Portland Transit Mall, and the red/blue MAX line downtown, are examples of this
- Level 2: Dedicated lanes, less frequent stops (2000-4000 feet). Generally adjacent to traffic, albeit with some physical separation; and featuring signal pre-emption. The Yellow Line along Interstate is an example for MAX. EmX in Eugene is an example of a level 2 BRT system.
- Level 3: Dedicated surface infrastructure, relatively infrequent stops (3000-5000 feet), infrequent and protected crossings. Usually has signal priority, especially if rail based (due to longstanding legal precedent). If rail-based, often features rails running on open ballast rather than in pavement. The Blue Line between Beaverton and Hillsboro is a good example of this. The LA Orange Line is a bus-based example, albeit without signal priority.
- Level 4: Mostly grade-separated, infrequent stops (4000-7000 feet), few crossings, full signal priority where crossings do occur. May have pedestrian crossings of the route at platforms. Quite a few parts of MAX meet this standard, including the stretch between Sunset and Goose Hollow, the Banfield segment, the Green Line south of Gateway, and the Red Line spur to the airport. Some busways, such as found in Ottawa and Brisbane, also meet this standard. "Metro lite".
- Level 5: Fully grade-separated, fully protected ROW, very infrequent stops. No pedestrian access to route whatsoever. Crossings are extremely rare if permitted at all. Suitable for driverless operation and third-rail power. Most subways and els meet this standard; the differences between this and 4 don't really apply to busses.
- Level X: Mixed-traffic running on high-performance routes, point-to-point service over longer distances. Several miles, typically, between stops. Express busses, commuter rail. WES is an example, as are the various "X" bus lines.