2N3904 is a NPN transistor hence the collector and emitter will be left open (Reverse biased) when the base pin is held at ground and will be closed (Forward biased) when a signal is provided to base pin. 2N3904 has a gain value of 300; this value determines the amplification capacity of the transistor. The maximum amount of current that could flow through the Collector pin is 200mA, hence we cannot connect loads that consume more than 200mA using this transistor. To bias a transistor we have to supply current to base pin, this current (IB) should be limited to 5mA.
