G.8.1- Counters are used to keep track of long-term changes to a permanent. [CompRules 1999/04/23]
G.8.2- Counters of the same name are interchangeable. For example, Poison counters from all sources are still Poison counters. [CompRules 1999/04/23]
G.8.3- Counters with just a value, such as +1/+1, are interchangeable with other counters with exactly the same value. [CompRules 1999/04/23] For example, a Spike Drone can move its counter onto a Tetravus and the Tetravus could use the counter.
G.8.4- Counters are considered to apply themselves to the permanent just after the initial characteristics of that permanent, and before any effects are applied. [CompRules 1999/04/23]
G.8.5- Counters remain on a permanent even if they currently do not apply. [CompRules 1999/04/23] For example, if a Mishra's Factory gets a +1/+1 counter while it is a creature, the counter will remain when it turns back into a land, and it will still be there if it later turns back into a creature. Similarly, a Licid will keep a +1/+1 counter even while it is in enchantment form.
Note- Also see Token Creatures, Rule K.25.