A microlaryngoscopy tube (also generally called 'MLT', although this is a registered trademark for Covidien Inc.) is in essence a pediatric-sized standard ETT with an adult length or an adult standard ETT with a pediatric-sized diameter, whichever way you want to look at it.
'MLT' = massively long tube
These tubes come in three sizes, 4.0, 5.0 and 6.0mm. The size 5.0mm tube is the most commonly used.
This picture shows two 5.0mm endotracheal tubes, the 'standard' one below, which is used in pediatric practice, and the microlaryngoscopy one above, which is of course longer and has a bigger 'adult' cuff.
Same diameter, different length, and shown here, different-sized cuff.
Microlaryngoscopy tubes are used, as the name suggests, in laryngeal surgery. Particularly in vocal chord surgery, the smaller diameter of these tubes allows the surgeon an easier view and access to the vocal chords and adjacent structures.
[Picture of MLT in situ versus standard tube]
One thing to keep in mind when using ETTs of such small size in adults is the relatively high resistance to gas flow through these tubes, meaning that comparatively high airway pressures are required to deliver the desired tidal volumes. Similarly, gas flow is slow during expiration, meaning that lower I:E ratios are often necessary to allow for complete expiration.