Graft

From MTG Salvation Wiki

Graft is a keyword ability, introduced in the Dissension expansion, that is associated with the Simic guild, and works by allowing a creature with graft to move a +1/+1 counter from itself onto a creature that just came into play. Thus far, only creatures that are aligned with the Simic guild (green and/or blue) have graft. The only non-creature card with graft is Future Sight's Llanowar Reborn.

From the Comprehensive Rules:

502.54 Graft
- 502.54a Graft represents both a static and a triggered ability. "Graft N" means "This permanent comes into play with N +1/+1 counters on it" and "Whenever another creature comes into play, you may move a +1/+1 counter from this permanent onto that creature."
- 502.54b If a creature has multiple instances of Graft, each one works separately.

[edit] Rulings

All creatures with graft in Dissension are 0/0 creatures. When the last +1/+1 counter is moved off of it, it will be put into its owner's graveyard as a state-based effect unless some other effect is boosting its toughness.
Graft's triggered ability triggers each time a creature comes into play, regardless of who controls the new creature. A player may move a +1/+1 counter onto another player's creature this way.
When a creature comes into play, all graft abilities will trigger. The controllers of those abilities choose, one at a time as those abilities resolve, whether to move a counter. A creature that has just come into play may receive multiple +1/+1 counters this way.
Graft isn't targeted. The ability can move a counter onto an untargetable creature or a creature with protection from creatures.
The counter moved off a creature with graft as the graft ability resolves doesn't have to be one of the counters put on the card as a result of the graft ability. It can be any +1/+1 counter on it. No other types of counters may be moved this way.
If a permanent with graft isn't a creature when a new creature comes into play, the graft ability will still trigger and a +1/+1 counter may still be moved.
If a creature with graft and another creature come into play simultaneously, the graft ability will trigger. The ability's controller may move a +1/+1 counter onto the other creature.
Many creatures with graft also have an activated ability that targets a creature with a +1/+1 counter on it. In most cases, that means it can target itself. The target creature may have any number of +1/+1 counters on it. The target creature doesn't need to have graft.
If the target of such an activated ability loses all its +1/+1 counters before the ability resolves, the ability will be countered for having an illegal target. If the targeted creature loses all its +1/+1 counters after the ability resolves, it won't lose the bonus it was granted by the ability.

[edit] Example

Helium Squirter -
Creature — Beast Mutant
0/0
Graft 3 (This creature comes into play with three +1/+1 counters on it. Whenever another creature comes into play, you may move a +1/+1 counter from this creature onto it.)
: Target creature with a +1/+1 counter on it gains flying until end of turn.