Absorption

In pharmacology (and more specifically pharmacokinetics), absorption is the movement of a drug into the bloodstream.
Absorption involves several phases. First, the drug needs to be administered via some route of administration (oral, via the skin, etc.) and in a specific dosage form such as a tablet, capsule, and so on.
In other situations, such as intravenous therapy, intramuscular injection, enteral nutrition and others, absorption is even more straight-forward and there is less variability in absorption and bioavailability is often near 100%.
Absorption is a primary focus in drug development and medicinal chemistry, since the drug must be absorbed before any medicinal effects can take place. Moreover, the drug's pharmacokinetic profile can be easily and significantly changed by adjusting factors that affect absorption.
The Processes by which the concentration of the drug at any moment and in any region can be determined is done by translocation of drug molecule. The drug is translocated in the body by bulk flow and diffusion. If the drugs chemically differ, still the transfer by bulk flow can occur by the same mechanism but if the drugs are moving by diffusion, it means that their movement is markedly different. The transfer of a drug is highly dependent on its solubility in either lipid or water.
For movement of the drug from the GIT to the system the sink condition is playing a vital role. Sink condition means, the drug is always in circulation due to blood circulation. So, the conc. of drug is not reaching at equilibrium. Thus, the drug can be diffused due to no equilibrium state.
The smaller molecules can move faster than larger ones.

Dissolution
In the most standard situation, a tablet is ingested and passes through the esophagus to the stomach. Because the stomach is an aqueous environment, this is the first place where a tablet will dissolve.
The rate of dissolution is a key target for controlling the duration of a drug's effect, and as such, several dosage forms that contain the same active ingredient may be available, differing only in the rate of dissolution. If a drug is supplied in a form that is not readily dissolved, the drug may be released more gradually over time with a longer duration of action. Having a longer duration of action may improve compliance since the medication will not have to be taken as often. Additionally, slow-release dosage forms may maintain concentrations within an acceptable therapeutic range over a long period of time, as opposed to quick-release dosage forms which may result in sharper peaks and troughs in serum concentrations.
The rate of dissolution is described by the Noyes-Whitney equation as shown below:

Where:
dW/dt is the rate of dissolution.

A is the surface area of the solid.
C is the concentration of the solid in the bulk dissolution medium.
Cs is the concentration of the solid in the diffusion layer surrounding the solid.
D is the diffusion coefficient.
L is the diffusion layer thickness.

As can be inferred by the Noyes-Whitney equation, the rate of dissolution may be modified primarily by altering the surface area of the solid. The surface area may be adjusted by altering the particle size (e.g. micronization). The rate of dissolution may also be altered by choosing a suitable polymorph of a compound. Specifically, cystalline forms dissolve slower than amorphous forms.
Also, coatings on a tablet or a pellet may act a barrier to reduce the rate of dissolution. Coating may also be used to modify where dissolution takes place. For example, enteric coatings may be applied to a drug, so that the coating only dissolves in the basic environment of the intestines. This will prevent release of the drug before reaching the intestines.
Since solutions are already dissolved, they do not need to undergo dissolution before being absorbed.

IonizationThe gastrointestinal tract is lined with epithelial cells. Drugs must pass through these cells in order to be absorbed into the circulatory system. One particular cellular barrier that may prevent absorption of a given drug is the cell membrane. Cell membranes are essentially lipid bilayers which form a semipermeable membrane. Pure lipid bilayers are generally permeable only to small, uncharged solutes. Hence, whether or not a molecule is ionized will affect its absorption, since ionic molecules are considered charged molecules by definition.
The Henderson-Hasselbalch equation offers a way to determine the proportion of a substance that is ionized at a given pH. In the stomach, drugs that are weak acids (such as aspirin) will be present mainly in their non-ionic form, and weak bases will be in their ionic form. Since non-ionic species diffuse more readily through cell membranes, weak acids will have a higher absorption in the highly-acidic stomach.
However, the reverse is true in the basic environment of the intestines-- weak bases (such as caffeine) will diffuse more readily since they will be non-ionic.
This aspect of absorption has been targeted by medicinal chemistry. For example, a suitable analog may be chosen so that the drug is more likely to be in a non-ionic form. Also, prodrugs of a compound may be developed by medicinal chemists-- these chemical variants may be more readily absorbed and then metabolized by the body into the active compound. However, changing the structure of a molecule is less predictable than altering dissolution properties, since changes in chemical structure may affect the pharmacodynamic properties of a drug.

Other factors
factors which affecting bioactivity, resonance, inductive effect, isosterism, bio-isosterism, spatial consideration.

Further reading:
Pharmacokinetics Made Easy, Revised by Donald Birkett

Basic Clinical Pharmacokinetics (Basic Clinical Pharmacokinetics (Winter)) by Michael E. Winter
Applied Clinical Pharmacokinetics by Larry Bauer