Digitalis is a plant that contains cardiac glycosides used to treat congestive heart failure and cardiac arrhythmias. Two main species are Digitalis lanata and Digitalis purpurea, which contain glycosides like digoxin, digitoxin, and others. These work by inhibiting the sodium-potassium ATPase pump, increasing cardiac contractility and slowing heart rate. Digitalis has a narrow therapeutic index and can cause side effects like nausea, arrhythmias, and toxicity if levels become too high due to interactions or improper dosing based on factors like renal function. Careful monitoring is needed when using this drug.