These two mechanisms are based on two possible behaviors for a carbocation formed by unimolecular dissociation of a C-X bond. Hence they always occur together, and therefore are only infrequently useful for synthesis.
The carbocation intermediate means that the usual stereochemical result is racemization. However, in relatively non-polar solvents, ion pairs may lead to an excess of inversion. In E1 reactions the more substituted alkene always predominates.
Here is a decision tree [CLICK] that summarizes our rules for deciding when SN2, SN1-E1, and E2 will occur.
SN1-E1 is facilitated by polar protic solvents that strongly solvate the leaving group and aid ion separation.
One important rule is that SN1-E1 NEVER occurs at primary carbons.