An enduro-styled motorcycle typically has long travel and medium-hard suspension of a motocross bike conjoined with features such as a headlight and quiet muffler to make the bike street-legal for parts of the track. The engine of an enduro bike is usually a single cylinder two stroke between 125cc and 360cc, or four stroke between 250 and 650cc.
There can be an advantage to having an engine size smaller than 650cc in some Enduro events as it is nearly always lighter, which means it has easier handling for getting around trees, obstacles, etc. However, in some Enduro events, the larger bike sizes allow them to get up hills without going down a gear or even two.
Enduro motorcycles most closely resemble Motocross (MX), and MX bikes are often selected by many riders for Enduro racing (albeit with significant modification, such as those described above). The demands that Enduros place on a bike are severe, and compared to standard street motorcycles, Enduro bikes require a great deal of maintenance to maintain peak operating capability. Their maintenance can be compared to that of Motocross bikes, except that Enduro bikes tend to require more collision prevention/repair maintenance and, until recently, Enduro bikes were developed by their manufacturers as totally different bikes than MX bikes (which is why many races rode/ride MX bikes in Enduro events), and accordingly they were developed with longer service-life engines, more durable components, less concern for weight (since, unlike MX, there are often no lower-limits on bike weights in a class), and other Enduro specific considerations.
The trend since the early 2000s, though, has been to use MX racing bikes as the platforms for Enduro bikes. This was partially driven by the conversion of MX from 2-stroke to 4-stroke engine designs to comply with regulatory trends. Enduro or so-called "Trail-bikes," or "Dual-sport" (dual because they are capable of moderate off-road and moderate street-performance, while excelling at neither) traditionally had a much higher proportion of 4-stroke models. The power-to-weight ratio of these 4-stroke trail bikes wasn't great by today's standards, and that had to do with the lack of emphasis on meeting a weight lower-limit. When the conversion to 4-stroke MX bikes began, there was suddenly an emphasis on power-to-weight ratios for off-road 4-stroke motorcycles, because 125 and 250 MX classes have minimum weight limits. Once these bikes entered production, the manufacturers decided that rather than inventing the next generation of Enduro type four stroke bikes, they would simply re-configure their existing 4-stroke MX lineup for Enduro. Accordingly, they made the above listed configuration changes.
The end result of all of this is that today's Enduro motorcycles, which are predominantly 4-strokes are lighter and more powerful than ever, but have many of the same characteristics of their MX cousins. These bikes, which were originally engineered to be MX bikes, do not often stand up to the rigors of long-term Enduro use. In the quest to achieve lower weight and higher power, the manufacturers made many compromises (i.e., thinner walls, lighter materials, more stressful designs, etc...), the downsides of which aren't always evident in MX where the races are relatively short and the bikes are thoroughly serviced after each event and replaced after every season for serious riders. In the case of Enduro riding, these compromises became painfully evident when the bikes were subjected to the harsh Enduro operating conditions. One such example is the 2004 Honda CRF250X Enduro bike, which was based on the CRF250R MX bike. The valves were notorious for needing to be constantly adjusted and even replaced in many cases. Neither procedure is trivial. This issue has apparently been rectified on the 2006 models, due to a reported valve-seat material change on the cylinder head. In contrast, the comparable Husqvarna TE250, which is purpose built for Enduro riding, is not only low maintenance by comparison, but the types of maintenance that are required more frequently on bikes that are used in Enduros are much easier to do because the bike was designed with those procedures as a design concern, such as valve checks and adjustments.