Components can declare their own storage variables. When a contract uses a component, the component storage is merged with the contract storage. The storage layout is only determined by the variables names, so variables with the same name will collide.
In a future release, the #[substorage(v1)] will determine the storage layout based on the component as well, so collisions will be avoided.
A good practice is to prefix the component storage variables with the component name, as shown in the Switchable component example.

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Example

Here’s an example of a collision on the switchable_value storage variable of the Switchable component. Interface: 
#[starknet::interface]
pub trait ISwitchCollision<TContractState> {
    fn set(ref self: TContractState, value: bool);
    fn get(ref self: TContractState) -> bool;
}
Here’s the storage of the contract: 
#[storage]
#[allow(starknet::colliding_storage_paths)]
struct Storage {
    switchable_value: bool,
    #[substorage(v0)]
    switch: switchable_component::Storage,
}
Both the contract and the component have a switchable_value storage variable, so they collide: 
#[test]
fn test_collision() {
    let (mut contract, mut contract_iswitch) = deploy();

    assert_eq!(contract.get(), false);
    assert_eq!(contract_iswitch.is_on(), false);

    contract_iswitch.switch();
    assert_eq!(contract_iswitch.is_on(), true);
    assert_eq!(contract.get(), true);

    // `collision` between component storage 'value' and contract storage 'value'
    assert_eq!(contract.get(), contract_iswitch.is_on());

    contract.set(false);
    assert_eq!(contract.get(), contract_iswitch.is_on());
}