The Indus script (also Harappan script) is a corpus of symbols produced by the Indus Valley Civilization during the Kot Diji and Mature Harappan periods between the 35th and 20th centuries BC. Most inscriptions are extremely short. It is not clear if these symbols constitute a script used to record a language, and the subject of whether the Indus symbols were a writing system is controversial. In spite of many attempts at decipherment, it is undeciphered, and no underlying language has been identified. There is no known bilingual inscription. The script does not show any significant changes over time..
More information on the Wikipedia page 
The Harappa website has a number of articles on the Indus valley script here