Reactions with alkynes are similar to those associated with alkenes because of the nature of the pi bond. The two pi bonds of alkynes are located orthoganol to each other and they surround the plane of the carbon-carbon sigma bond. Therefore, the electrons located in the pi bonds create an area of electron density that are vulnerable to reactions with electrophiles. This is represented by the red area on the electrostatic potential surface diagram. 

Terminal alkynes (alkyns with at least one H atom on an sp hybridized carbon) can be deprotonated in the laboratory using a strong base such as NaNH₂. Terminal alkynes are relatively acidic because the anion produced places the lone pair of electrons, and thus negative charge, in an sp orbital. An sp orbital is 50% s character (and 50% p character), and since s orbitals are lower in energy than p orbitals, an sp orbital is relatively low in energy. As a result, the alkyne anion is relatively stable compared to other types of carbon anions (ex. sp² or sp³) and terminal alkynes are threfore relatively acidic (pKa ~25). (Recall that generally the more stable the conjugate base anion, the more acidic the parent acid). Terminal alkyne anions can react as VERY STRONG nucleophiles with primary or methyl alkyl halides to diplace the halide and create a NEW CARBON-CARBON BOND (HURRAY!!!). The reaction takes place in a single step, with one transition state and no intermediates. You will see later that secondary or tertiary alkyl halides give mostly "elimination" products (namely alkenes), not substitution products with new carbon-carbon bonds.