What is a foot fetish?
A foot fetish is when a person is sexually aroused by viewing, handling, licking, tickling, sucking, sniffing or kissing the feet and/or toes of another person, or by having another person doing the same to their own feet. The fetishist response to the foot is the same or very similar to arousal at seeing genitals (or the sex-specific features like female breasts). Others enjoy having their genitals manipulated by their partner's feet to the point of orgasm or ejaculation (which is called a "footjob", analogous to the concept of a "handjob"), or merely being touched by feet or used as a foot-rest by another person.

Like other paraphilias, foot fetishism varies widely and can be highly specialized, therefore one fetishist may be aroused by elements another might find repulsive. Some foot fetishists prefer to concentrate on feet that are arched, others concentrate on relaxed feet. Most prefer bare feet, while others may prefer feet dressed in certain types of shoes, socks or hosiery. Some even concentrate on specialized scenarios, such as bare feet resting on a car dashboard. Some like to bite toenails. Thus there is often a blurred distinction between foot, nylon, pantyhose, shoe, dirt and similar fetishes.
A foot fetishist can be male or female (although males account for over 70%), and can have an interest in the feet of either sex, albeit usually parallel to their sexual orientation. Other fetishes such as tickling or bondage often accompany foot attraction.
Some theories on how foot fetishism forms are as follows:
One explanation, advanced by neurologist Vilayanur S. Ramachandran, highlights the fact that the feet and the genitals occupy adjacent areas of the sensory cortex, possibly entailing some neural crosstalk between the two.

A second guess is that in many cultures, an infant's experience of its mother may involve crawling around her feet. If sexual/behavioral imprinting occurs during this time, this might lead to the feet becoming the primary object of sexual arousal.
Another theory is that the shape of the human foot is distinctly phallic from certain angles and replicates the shape of either male genitals (explaining homosexual attraction to male feet), or the curves of the female body (again explaining the tendency of heterosexual foot fetishists to be men).
And finally the last theory proposes that visually, the feet and genitals occupy the same space within the sexual visual field that a male sees of a female, when the legs are raised for sex, and that some males are genetically brain imprinted to find parts of the body within that field as erotic.
So you have a foot fetish - now what?
Foot worship is the practice in which a woman or a man gives extreme adulation to the foot of another person. It is practiced by both straight and gay couples, especially those who participate in BDSM and fetish activities.
Foot worship can include licking the foot of one's "dom" or "domme", massaging it, kissing it, praising it, allowing oneself to be stomped or trampled by it, etc. Others gain sexual pleasure from the odors present from sweaty feet, and often unclean socks and footwear form part of the foot worship activity. Thus, the term "worship" itself really has nothing to do with the normal interpretation of the word "worship" in the English language. It's actually more akin to "making love to" or "foreplaying with" the dom's foot.

Typically the enjoyment of foot worship stems from the fact that the foot is usually considered one of the "lowest" and least appreciated parts of the body; thus if a "sub" worships the foot of a "dom" then the sub acknowledges that her or his status is so far below that of the "dom" that she or he worships even the least appreciated part of her or his dom's body. Foot worship can be historically linked to the practices in various ancient or traditional cultures where a person (from a "lower" social stratus such as a slave or peasant) shows adulation by kissing the feet of another (from a "higher" social stratus such as a king or queen). The same may be said, by extension, of boot worship etc. "Worshipping the ground the dom walks on" is another variant of foot worship. In this case, the slave is made to kiss or lick parts of the floor that the dom has stepped on. Some women and men who enjoy giving foot worship have admitted that the act of kneeling and lowering her or his head (in order to perform the worship), thereby acknowldging her or his unworthiness before her or his "dom", is actually equally arousing to the act of kissing or licking itself.
Like many other "kinky" sexual activities, there is also some enjoyment derived from the fact that one is indulging in a taboo activity by "worshipping" the foot of another person.