There are four species that are often found near homes:

Common Garter Snake: Size: 2–4 1/2 feet, usually smaller
Eastern Milk Snake: Size: up to 4 1/2 feet long. Sometimes confused for the copperhead snake
Black Rat Snake: Size: up to 4 1/2 feet long
Northern Water Snake: Size: up to 4 1/2 feet long.

There are 17 species of snakes in New York. Eight of them are statewide.

Listed below are the species found throughout the state, and the rarest species, because three of them receive special legal protection.
New York is home to two endangered snakes (Eastern massasauga and queen snake), one threatened species (timber rattlesnake), and two species of special concern (eastern hognose snake and eastern worm snake). Although the category, “species of special concern” does not give any extra protection, it does show that the population is low enough to worry biologists.

Nonvenomous snakes, usually Docile:
Found statewide:
Common garter snake, Thamnophis sirtalis. Common near homes.
Eastern milk snake, Lampropeltis t. tiiangulum. Common near homes.
Northern brown snake, Storeria d. dekayi
Smooth green snake, Liochlorophis vernalis
Northern redbelly snake, Storeia o. occipitomaculata
Eastern ribbon snake, Thamnophis sauritus
Northern ringneck snake, Diadophis punctatus edwardsii

Found only in certain parts of the state:
Black rat snake, Elaphe o. obsoleta
Found only in scattered pockets, upstate; Common near homes.
Queen snake, Regina septemvittata.
Rare, in scattered pockets in western New York; Endangered.
Eastern hognose snake, Heterdon platirhinos
Coastal plains and Hudson River Valley; Species of special concern.
Eastern worm snake, Carphophis a. amoemus
Coastal plains and north to Albany county; Species of special concern.
Shorthead garter snake, Thamnophis brachystoma
Southern tier only

Nonvenomous, Aggressive:
Found Statewide:
Northern water snake, Nerodia s. sipedon.
Common around homes with nearby ponds.

Venomous, Defensive:
Found only in certain parts of the state:
Eastern massasauga, Sistrurus c. catenatus
Rare, in Onondoga and Genessee counties only; Endangered.
Timber Rattlesnake, Crotalus horridus
Rare, in lower Hudson Valley, parts of western New York, and the southern Adirondacks; Threatened.
Northern copperhead, Agkistrodon contortrix mokasen
Lower Hudson Valley)

How to tell a Nonvenomous snake from a Venomous snake:
Nonvenomous snake:
Pupil Round
No pit between the eye and nostril
Shape of head is variable, often slender
Scales underneath the tail, toward the tip, are divided

Venomous snake:
Pupil: like a cat’s eye, vertical
Pit between the eye and nostril; (the 3 venomous snakes in NY are all pit vipers)
Broad, triangular-shaped head
Scales underneath the tail, toward the tip, are undivided
Signs of their presence:
Water snakes often sun themselves on boat docks.

Milk snakes and black rat snakes are frequently found in barns.

On cool days, you may find snakes (especially the black rat snake) resting on top of the heating ducts in buildings heated with forced hot air.

Sounds: Silent, except for the rattlesnake, which rattles, and the milk snake, which may also vibrate its tail if annoyed.

Scat: Elongated, whitish. The scat of a black rat snake may be large.

Large shed skin (over 2 foot long): Probably from a black rat snake.

Evidence of their feeding: Hard to identify, because they swallow their prey whole.

Garden and crop damage: None, because they are strictly carnivores.

Common nuisance situations:

Time of year: Spring through fall.

Snakes don’t damage buildings or eat crops. They only enter buildings through existing holes, cracks or “doors” (such as an open window). Some people are afraid of snakes. Others welcome them, because some snakes eat mice and rats and help to control those pest populations. Remember, however, that if a snake can get into a home, so can other creatures.
These snakes sometimes hibernate in buildings, especially the basements of old houses with stone foundations. They usually enter houses through torn screens, open basement windows, cracks in the foundation, or through gaps next to pipe and cable entrances.

They follow prey (mice, insects) into cellars, crawl spaces, attics, barns, sheds, garages. They may also be found in wood piles and debris, in heavily mulched gardens, and under shrubs, tarps or planks. They seek cool, damp, dark places.

Their presence may frighten or annoy people. Several species, including the garter snake, may emit a foul and musky smell when handled.

Disease risks: salmonellosis (food poisoning).

Injury risks: nonvenomous snakes have tiny teeth. They leave a faint, U-shaped bite mark. Their bites rarely hurt much or cause problems, with the exception of the northern water snake, which is known for its nasty bite. Few people encounter New York’s venomous snakes, and fewer still are bitten—and even then, the bites are rarely fatal. A bite from one of New York’s venomous snakes (copperhead, massasauga, timber rattlesnake) will swell, hurt, and turn black and blue. Children and the elderly are at greatest risk for a severe reaction. If bitten, remain calm and get medical help. Do not use a commercial snake bite kit; they tend to do more harm than good.
For more information:
http://www.nwco.net/08-AppendixBNuisanceSpecies/8-15-Snakes.asp