A receptor is a molecule that binds a signaling ligand and couples that recognition event to a cellular response. A signaling ligand can act through a cell-surface receptor or an intracellular receptor, depending largely on whether the ligand can cross the plasma membrane and where the receptor's functional domain operates.

Cell-surface receptors

Many signaling molecules are polar, charged, or too large to cross the hydrophobic core of a lipid bilayer efficiently. These ligands bind receptors exposed on the cell surface.

Ligand binding changes the receptor's conformation or association state. That change is transmitted to the cytoplasmic side, where the receptor can alter enzymes, ion channels, adaptor proteins, or other intracellular components.

extracellular ligand
       ↓ binds
transmembrane receptor
       ↓ conformational change
intracellular signaling machinery

The ligand itself does not need to enter the cell.

Intracellular receptors

Small hydrophobic ligands can often cross lipid bilayers. Some bind receptors in the cytosol or nucleus.

Many intracellular receptors can regulate transcription, the production of RNA from DNA. Ligand binding changes the receptor's ability to interact with DNA or with proteins that control transcription.

membrane-permeable ligand
        ↓ crosses membrane
intracellular receptor
        ↓
change in transcription

Receptor location changes response mechanism

Cell-surface pathways can modify proteins already present in the cell and therefore can produce rapid responses. Intracellular receptors commonly change transcription and can produce longer-lasting changes because the cell's molecular composition can subsequently change.

These are tendencies rather than rigid rules: surface-receptor pathways can also alter transcription.

The key distinction is mechanistic: membrane permeability determines whether an extracellular ligand can reach an intracellular receptor directly, while receptor location determines how signal recognition is coupled to cellular machinery.