The four states |00⟩, |01⟩, |10⟩, |11⟩ form a basis so
any two-qubit state can be written as a linear combination of these vectors: ∣ψ⟩ = a|00⟩ +b|01⟩ +c|10⟩ + d|11⟩. If a certain coefficients in the linear combination are zero, e.g., b and c for the 00+11 state, or a and d for the second one, this doesn't have any special meaning.
The classification of states as separable vs entangled refers to the existence of a local description for the two qubits. Remember that |00⟩ is shorthand for |0⟩⊗|0⟩, meaning the state of the two-qubit system when qubit 1 is in state |0⟩ and qubit 2 is in state |0⟩.
Separable states can be written in the form (α|0⟩+β|1⟩)⊗(γ|0⟩+δ|1⟩) = ∣ψ1⟩⊗∣ψ2⟩. Note there is a clear local description for the first qubit ∣ψ1⟩ and and a separate local description of the state ∣ψ2⟩. If Alice prepares her qubit 1 in the state ∣ψ1⟩ and Bob prepares his qubit 2 in the state ∣ψ2⟩ then the combine description of their two qubits is what's shown above. The state of the combined system is describable as the tensor product of two separate local descriptions.
Entangled states, on the contrary, are states that cannot be described as the tensor product of two local descriptions. Specifically, there exist configurations a,b,c,d for a two-qubit quantum system such that
a|00⟩ +b|01⟩ +c|10⟩ + d|11⟩ ≠ (α|0⟩+β|1⟩) ⊗ (γ|0⟩+δ|1⟩)
no matter what choice of α,β,γ,δ you make. The phenomenon of entanglement is a quantum-only thing, that can't really be understood via classical analogies, since classical systems can necessarily be described as the combination of two local descriptions. The examples given are two of the four Bell states, see
https://en.wikipedia.org/wiki/Bell_state , but there are many more entangled states. Here is a concrete physics example
https://en.wikipedia.org/wiki/Singlet_state#Singlets_and_Ent...Interestingly, many of the quantum computing experiments perform involve the manipulation of entabgled states because they serve as proof that something quantum is going on...