An oscilloscope (often abbreviated o'scope or scope) displays signal voltages as a 2-D graph, usually as voltage (vertical axis) as a function of time (horizontal axis).
The probe of a o'scope is held against, or is clipped to, the metal wire carrying the signal of interest.
A dual-trace o'scope has 2 probes, so 2 different signals can be seen -- so not only the shape of the signal can be seen, but also the exact time from events on one signal to events on the other signal.
Sometimes one wants to look at a 8-bit data bus -- typically one uses a "logic analyzer" to look at lots of time-aligned digital signals. For a given number of signals to look at, oscilloscopes (which show the true analog waveform) are much more expensive than logic analyzers (which only show if a digital waveform is hi or low, above or below the logic threshold). Many people buy both a (dual-trace) o'scope and a 16-trace logic analyzer, which together have a total cost less than one true 8-trace o'scope.
... more techniques ...
some low-cost oscilloscopes
some low-cost oscilloscopes, in no particular order.
- price name comments
- $420 PicoScope 2202 from Pico Technology()()
- $200 Hantek DSO-2090 USB PC Oscilloscope 
- $380.00 DSO2250 100MHz PC-SCOPE 
- CAN 219.99 Syscomp Electronic Design DSO-101 
- EUR 427 ETC s.r.o. M520 Oscilloscope 
- $222 ANT8 : 8 Channel Logic Analyzer (EasySync)
- $200 PS40M10 : Swordfish Hand-Held Instrument (1 channel) USB (EasySync)
- $220 DS1M12 : Stingray Multi-Function Instrument (2 channel) USB (EasySync)
- under $600 : Jameco has some actual stand-alone oscilloscopes for under $600.
- under $200 : Carl's Electronics has some stand-alone and PC-hosted (parallel port?) oscilloscopes for under $200.
EasySync is a distributor of RockyLogic's ANT8/etc products.
- Keith has made a list of "PC USB logic analyzers that cost under $1000.", some of which can be used as an o'scope. Should I use the same $1000 price cut-off for the above list? Or perhaps reduce it to $500 ?
- Brooke Clarke has posted a list of "PC based Oscilloscopes"
- Wikipedia: oscilloscope
- Other discussions of "Cheap digital storage oscilloscope"