Sushi rice is cooked from short grained or medium grained rice, which is naturally stickier than long grain rice.
Also, rice used to make sushi is coated with with a mixture of rice wine vinegar, salt and sugar... sometimes mirin (sweetened Japanese rice wine) so the rice has a vinegary, sweetness to it
From the link below...
Sushi Rice Recipe - Japanese Recipe
Japanese rice is a medium-grained rice and gets sticky when it is cooked. The long-grained American rice isn't proper for sushi because it is drier and doesn't stick together.
INGREDIENTS:
3 cups Japanese-style rice
3 1/4 cups water
1/3 cup rice wine vinegar
3 tbsp sugar
1 tsp salt
PREPARATION:
After washing the rice well, cook it by pan or rice cooker.
Prepare sushi vinegar (sushi-zu) by mixing rice vinegar, sugar, and salt in a small pan.
Put the pan on low heat and heat until the sugar dissolves. Cool the vinegar mixture.
Spread the hot steamed rice into a large plate (if you have it, use a wooden bowl called sushi-oke) by shamoji (spatula).
Sprinkle the vinegar mixture over the rice and fold the rice by shamoji very quickly. Be careful not to smash the rice.
To cool and remove the moisture of the rice well, use a fan as you mix sushi rice. This will give sushi rice a shiny look. It's best to use sushi rice right away.
Makes 4-6 servings.