S_N1 indicates a substitution, nucleophilic, unimolecular reaction, described by the expression rate = k [R-LG].
This implies that the rate determining step of the mechanism depends on the decomposition of a single molecular species.

This pathway is a multi-step process with the following characteristics:
 

		step 1: slow loss of the leaving group, LG, to generate a carbocation intermediate, then
		step 2 : rapid attack of a nucleophile on the electrophilic carbocation to form a new s bond

	Multi-step reactions have intermediates and a several transition states (TS). In an SN1 there is loss of the leaving group generates an intermediate carbocation which is then undergoes a rapid reaction with the nucleophile..
General case		SN1 reaction

Lets look at how the various components of the reaction influence the reaction pathway:

R-
Reactivity order :   (CH₃)₃C-  >  (CH₃)₂CH-   >  CH₃CH₂-  >  CH₃-

In an S_N1 reaction, the rate determining step is the loss of the leaving group to form the intermediate carbocation. The more stable the carbocation is, the easier it is to form, and the faster the S_N1 reaction will be.  Some students fall into the trap of thinking that the system with the less stable carbocation will react fastest, but they are forgetting that it is the generation of the carbocation that is rate determining.

The following images show a selection of alkyl bromides and their relative rates of  reaction in an S_N1 hydrolysis.
Try to correlate the structure of the alkyl bromide with the type of carbocation that will be formed.
If you need help, click the L button to show you where the carbocation will be formed.
 

-LG
The only event in the rate determining step of the S_N1 is breaking the C-LG bond. Therefore, there is a very strong dependence on the nature of the leaving group, the better the leaving, the faster the S_N1 reaction will be.

Nu
Since the nucleophile is not involved in the rate determining step, the nature of the nucleophile is unimportant in an S_N1 reaction. However, the more reactive the nucleophile, the more likely an S_N2 reaction becomes.

Stereochemistry
 

In an SN1, the nucleophile attacks the planar carbocation. Since there is an equally probability of attack on each face there will be a loss of stereochemistry at the reactive center as both products will be observed.

Solvent
Polar solvents which can stabilize carbocations which can favour the S_N1 reaction (e.g. H₂O, ROH)

Since a carbocation intermediate is formed, there is the possibility of rearrangements (e.g. 1,2-hydride or 1,2-alkyl shifts) to generate a more stable carbocation. This is usually indicated by a change in the position of the alkene or a change in the carbon skeleton of the product when compared to the starting material.

This pathway is most common for systems with good leaving groups, stable carbocations and weaker nucleophiles. A typical example is the reaction of HBr with a tertiary alcohol.
 
 

SN1 MECHANISM FOR REACTION OF ALCOHOLS WITH HBr Step 1: An acid/base reaction. Protonation of the alcoholic oxygen to make a better leaving group. This step is very fast and reversible.  The lone pairs on the oxygen make it a Lewis base.        Step 2: Cleavage of the C-O bond allows the loss of the good leaving group, a neutral water molecule, to give a carbocation intermediate. This is the rate determining step (bond breaking is endothermic)          Step 3: Attack of the nucleophilic bromide ion on the electrophilic carbocation creates the alkyl bromide.

SN1 MECHANISM FOR REACTION OF ALKYL HALIDES WITH H2O Step 1: Cleavage of the already polar C-Br bond allows the loss of the good leaving group, a halide ion, to give a carbocation intermediate. This is the rate determining step (bond breaking is endothermic)     Step 2: Attack of the nucleophile, the lone pairs on the O atom of the water molecule, on the electrophilic carbocation creates an oxonium species.          Step 3: Deprotonation by a base yields the alcohol as the product.    Note that this is the reverse of the reaction of an alcohol with HBr. In principle, the nucleophile here, H2O, could be replaced with any nucleophile, in which case the final deprotonation may not always be necessary.