As more processing power and data storage is included in oscilloscopes, the distinction is becoming blurred. Mainstream oscilloscope vendors manufacture large-screen, PC-based oscilloscopes, with very fast (multi-GHz) input digitizers and highly customized user interfaces.
Software for a PC may use the sound card or game port to acquire analog signals, instead of dedicated signal acquisition hardware. However, these devices have very restricted input voltage ranges, limited precision/resolution, and very restricted frequency ranges. The ground reference for these inputs is the same as the ground for the PC logic and power supply; this may inject unacceptable amounts of noise into the circuit under test. However, these devices can be useful for demonstration, hobby use, or specific setups where these factors won't interfere. Ground reference can also be eliminated with capacitor AC coupling or a signal transformer.
If a sound card is used, frequency response is usually limited to the audio range, and DC signals cannot be measured without hardware modification. The number of inputs is limited by the number of recording channels and the inputs can handle only audio line-level voltages (usually ~1 Vpp) without the risk of damage.
If the game port is used as the acquisition hardware, the possible sampling frequency is very low, typically below 1 kHz, and the input voltages can only vary over a range of a couple of volts. In addition, the game port cannot easily be programmed for a specific sampling rate, nor can it be easily assigned a precise quantization step. The analog to digital conversion is accomplished by triggering the discharge of a capacitor and then measuring how long it takes to charge it to a fixed threshold that is seen as a "0" to "1" transition on the PC ISA bus. This means a huge resistance at the input takes longer to measure than a low resistance, which results in asymmetrical sampling intervals. These limitations only make it suitable for low-precision visualization of low frequency signals.