On July 01, 1991, Finnish Prime Minister, Harri Holkeri dialed the first ever call on GSM aka Global System For Mobile Communications. The call was made using Nokia hardware that used 900 MHz band. Since then GSM has grown exponentially and today, over 1.5 billion people in over 212 countries stay connected with the GSM technology. In 20 years, GSM has captured over 80% of the mobile market.
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GSM was the first technology to offer fully digital cellular system using TDMA which crammed more information in the narrower spectrum and offered better call qualities using devices that consumed very little power. GSM also pioneered the concept of SIM (Subscriber Identity Module) cards so that users could easily switch the mobile devices without changing their number. The technology also offered international roaming. Even the most popular way of communication, SMS was first launched on GSM and the technology evolved to support EDGE & GPRS - which most of us have used to browse the Internet. Over the period of time, the technology has evolved to support 400, 800, 1800 and 1900MHz bands and now supports WDCMA-based UMTS (3G) and later HSPA and HSPA+.

Via: Nokia