A martial art is the practise of any exercise where the goal is to defeat an opponent through the application of physical force directed against them.
There are many martial arts, it's the very broadest of terms, but they can often be broken down further into more specific categories. Such as combat sports. They are still martial arts, but the emphasis of the training is the defeat of an opponent within a controlled (i.e. there are rules), competitive environment.
Traditional martial arts were historically practised to defeat opponents in an uncontrolled environment (i.e. there are no rules), so the emphasis of the training and types of techniques which are successful are often entirely different.
Modern martial arts are those skills, techniques, tactics and strategies practised today to defeat modern opponents through the application of physical force and usually in an uncontrolled environment. Armies and police forces the world over make use of modern martial arts, in differing contexts.
Martial arts also mutate over time. With constant usage as originally intended they remain as efficacious as they would have been in the past. However when an art becomes a combat sport, the techniques, tactics and strategies which are used to win the contest differ markedly from those needed to survive violent uncontrolled encounters, so the art changes over time. One of the best examples of this is wrestling. While within a sporting ring, wrestlers are very formidable, outside the ring in an uncontrolled environment many of the techniques and strategies used are today of dubious suitability.
Even without the deliberate switch to combat sport, when the directing influence of the violent environment is removed, the understanding of violence is eventually lost, the original art changes and so techniques are forgotten, misunderstood, poorly trained. The most extreme examples lose the martial component entirely and become traditional physical arts. You can see examples of this process in progress with various arts which are over time becoming less and less martial; Tai Chi, Aikido are examples where the emphasis is generally moving away from the martial aspect, and indeed, arts which appear to have lost all martial components entirely. Examples being Highland dancing, Morris dancing.
The efficacy of all martial arts should be considered suspect where they are taken out of context and where they are not being used regularly within an uncontrolled martial context.
Is Krav Maga a hybrid martial art? No, all martial arts are "hybrids" of earlier collections of strategies, tactics and techniques... it's not a useful term. It's simply a martial art; a name given to a collection of certain practises. A modern one rather than traditional, but one where the "use in anger" is relatively recent compared to others so less time has passed for it to mutate.