y(t) is likely continuous and y_t , the subscript form, is likely the discrete version. For example it is common in digital devices to sample something continu

y(t) is likely continuous and y_t , the subscript form, is likely the discrete version. For example it is common in digital devices to sample something continuous(analog) every unit of time and use whatever value was captured at that moment for whatever the goal is, ideally if the something is sampled frequently enough you can even recreate the continuous version from the values.