Conformal fuel tanks (CFTs) are additional fuel tanks fitted closely to the profile of an aircraft which extend either the range or "time on station" of the aircraft, with a reduced aerodynamic penalty compared to external drop tanks.
Conformal fuel tanks have the disadvantage that, unlike drop tanks, they cannot be discarded in flight. This is because conformal tanks are plumbed into the aircraft, and can only be removed on the ground. Thus, when the CFTs are empty, the aircraft continues to suffer the same aerodynamic penalty, with no benefit. However, to use the Eurofighter Typhoon as an example, an aircraft with CFTs can carry a full weapons load and two 1,500 litre CFTs, whereas a Typhoon without CFTs has to sacrifice at least two weapons pylons to carry drop tanks, reducing either range or weapons load.
CFTs also have the advantages of not significantly increasing an aircraft's radar cross-section, and allowing a higher maximum speed than drop tanks.
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