Can anyone tell me what the difference between a
hall effect sensor
linear hall effect sensor
ratiometric hall effect sensor
please.
Thanks
Linear and ratiometric are the same thing, output is proportional to applied magnetic field.
Digital output hall sensors are, latching, unipolar, omnipolar and bipolar.
Thanks for the clarification... I guess its a linear, or ratiometric I want then,, I guess I will get the least expensive 
What are you planning on using it for? All are not created equal, maybe read a few datasheets before purchasing.
"least expensive" might not offer what you need.
currently i have no real plan, aside from playing testing, getting to know limits and abilities... Yes I will read some specs first, thanks... I have no experience with any of this stuff. So its all about getting experience.
Linear are analog: powered up, the one I was looking at gives 0.5 of the input voltage as output,. The output goes up or down to supply volts or down to 0, with the presence of a S or N field.
I found my sensor wasn't reliable for speed measurements, wasn't "digital enough".... Only later did I read the data sheet. That was an Allegro 1302.
By contrast, the Allegro 110x are switches, and are active in the presence of a S pole.
Then there's the Diodes Inc AH180 which is also a switch, but not fussy about polarity, so it fires for a N or S pole. I'm looking for one of these, since it's easier if one doesn't have to worry about which way round the magnet's stuck on the rotor.
So various flavours for various purposes: I'd guess that for "messing around" a switch rather than linear would be better. I reckon one only worries about the strength, as opposed to presence, of a field in specific circs.
EDIT: This Allegro App Note might be useful.
Thanks Jimbo.. good info