In Inductive effect due to the EN difference between the adjacent atom or group, polarisation of sigma bond occurs, which causes a partial + and - charge on them. It is a permanent effect, which is effective upto 4-6$C$ only.
+I: polarisation occurs away from the atom or group, i.e., the adjacent group attached will act as electron pusher or donor or will have EN less than the carbon.
Example
$\ce{CH3-(C=O)OH }$, $\ce{-CH3}$ will acquire a partial + charge and $\ce{-(C=O)OH }$ will acquire partial - charge.
-I: polarisation occurs towards from the atom or group, i.e., the adjacent group attached will act as electron puller or acceptor or will have EN more than the carbon.
Example
$\ce{CH3-CH2-NO2}$, $\ce{-CH2-CH3}$ will have partial + charge and $\ce{-NO2}$ will have partial - charge.
As far as Lewis base is concerned its generally an electron pair donor, which makes it electron pucher most of the time , so, such lewis base would show +I effect.