The electrons around a nucleus will circulate in a magnetic field and create a secondary induced magnetic field. This field opposes the applied field as stipulated by Lenzs law and atoms with higher induced fields (i.e., higher electron density) are therefore called shielded, relative to those with lower electron density.  Electron-donating alkyl groups, for example, lead to increased shielding whereas electron-withdrawing substituents such as nitro groups lead to deshielding of the nucleus. Not only substituents cause local induced fields. Bonding electrons can also lead to shielding and deshielding effects. A striking example of this is the pi bonds in benzene. Circular current through the hyperconjugated system causes a shielding effect at the molecules center and a deshielding effect at its edges. Trends in chemical shift are explained based on the degree of shielding or deshielding.
Nuclei are found to resonate in a wide range to the left (or more rare to the right) of the internal standard. When a signal is found with a higher chemical shift:
* the applied effective magnetic field is lower, if the resonance frequency is fixed (as in old traditional CW spectrometers)
* the frequency is higher, when the applied magnetic field is static (normal case in FT spectrometers)
* the nucleus is more deshielded
* the signal or shift is downfield or at low field or paramagnetic
Conversely a lower chemical shift is called a diamagnetic shift, and is upfield and more shielded.