Thing is, they might be the same thing.
A19 refers to the shape and size of the bulb itself. "A" stands for Arbitrary -- just the name of the shape that's familiar to most of us. "19" refers to the size -- in this case, 19/8" (or 2 3/8") across.
E26 refers to the type and size of the base -- the part that goes into the light socket. "E" refers to "Edison," which is the type of "screw in" style base people in the United States are most familiar with. 26 refers to 26 mm across. (This is also called a "medium" base.)
So the light bulb that most of us have used in our homes here in the States is an A19 bulb with an E26 base.
You can find LED bulbs that fit that description here, or other types of bulbs here.
The reason it's useful to know both the shape and base is because you can't just ask for one of these and get the bulb you're looking for. Most A19 bulbs in the States will have an E26 base, but there are exceptions. And there are many shapes of bulbs that have E26 bases, so just asking for an E26 bulb won't get you very far.
We hope this helps you sort through the terminology!