The distinctive difference between a pictogram and an ideogram lies in the words themselves. A pictogram uses a picture of an object and an ideogram uses a symbol made of geometric shapes to represent an idea. Pictogram, picture. Ideogram, idea.
Examples of Pictograms
A picture of a lightning bolt means electricity or lightning. A picture of a cloud means rain. A picture of a flame means fire. A picture of a bottle means beverage. A picture of an airplane means airport. A picture of a knife and fork means restaurant. A picture of a man means men's room. A picture of a woman means ladies' room.
Notice that all of these represent concrete objects that you can touch, smell, taste, hear or see. For this reason anyone in his right senses can instantly understand the meaning of a pictogram or at least get a good guess by thinking about it.
Examples of Ideograms
Ideograms are more complex because they represent abstract or intangible ideas. They don't appear naturally as sensible objects in our environment. "No" is an idea, not a thing. Ideograms, therefore, must be agreed upon by consensual opinion and are expressed through geometry with lines and shapes. Someone must know through education what the meaning of ideograms are even though they are simple.
A circle with a line through it means no. An arrow means direct your attention this way. A figure eight on its side means infinity. Two horizontal lines parallel to one another means equal.
Combining Pictograms and Ideograms
A combination of pictures of objects and geometric shapes is the most common way pictograms and ideograms are used today. Most people use the terms interchangeably. A "no dogs allowed" sign using a picture of a dog in red circle with a line through it is an example of this combination. Is it a pictogram or an ideogram? You could use either term and be correct.
Ancient Use of Pictographs and Ideograms
There is nothing new in our use of pictograms and ideograms. Egyptians, Sumerians, Aztecs, American Indians and even cavemen used pictograms and ideograms to communicate and preserve knowledge. For archeologists trying to interpret ancient symbols, pictograms are easy to understand because they can be assumed to represent the object depicted, or at least ideas associated with it. Ideograms are far more difficult to read because the cultural meaning behind these shapes died with the people who created them.