While in the languages, such as English, French, Spanish, or Polish, a single letter or character defines just a separate sound (consonant or vowel), there are languages, where a single character can denote the whole word or concept. The examples of such languages are Chinese, Japanese, and Korean. The symbols, which are used in the written forms of those languages, are commonly referred to as ideographs.
Sometimes the word ideogram is used instead of ideograph. Actually, there is a certain amount of uncertainty in regard of these terms, obviously stemming from different misconceptions and misunderstandings of the Chinese language system. Modern scientists say that all the Chinese characters should be called logograms, while the term ideograph can be applied only to part of them.
Basically, the traditional Chinese lexicography divides all the characters, used in the language, into six categories, and ideographs form only one of them. The other categories are pictograms, ideogramic compounds, rebus characters, phono-semantic compound characters, and derivative cognates.
Speaking about the ideograph concept, it is interesting to note that the meaning of ideographs is often expressed through an iconic view, picture, or image of the object. For example, an ideograph, meaning "one", is simply a single horizontal line. While in order to define "two" or "three" you will have to draw two or three horizontal lines respectively. The ideograph 上 means "up", while in order to denote "below", you just have to put the same symbol upside down: 下.
Very commonly a Chinese character is called Han ideograph. Actually, this term is used at font-related web site Fonts2u, where the necessary typefaces can be found by specifying a particular character map. For example, looking for a font for Chinese fonts, one can select Han ideographs as the preferred character map and quickly receive the list of fonts, supporting the necessary characters.