Pawns are very important fighting units. While having limited mobility and being the least valuable piece, a pawn has a large potential and, together with other pawns, defines the course of the game. Pawns form the landscape of the battleground, and can be either weak or strong.
In most cases by strong pawns we mean:
1) Passed pawns
A passed pawn has no enemy pawns on the adjacent files, or they do not control the squares which the pawn has to pass along the way to promotion. Naturally, passed pawns are often a valuable asset, especially in the endgame.
2) Connected pawns
Pawns of the same color placed on adjacent files and able to protect each other. Such pawns are harder to stop since they don’t need protection from other pieces and are quite self-sufficient on the way to becoming a queen.
Of course, there are exceptions to these rules. In some situations even passed or connected pawns may be weak.
A pawn’s weakness originates from the fact that it can move only forward and attack just two (or even one) square. The most common types of weak pawns are:
1) Double (triple) pawns
Two pawns of the same color placed on the same file. Such pawns can’t protect each other and are even more limited in mobility. Naturally, there are exceptions when such pawns may become strong. For example, well-protected by other pieces double pawns that control the center can be a major force.
2) Pawn islands
Single pawns scattered all over the board. In the endgame they often become easy targets for the opponent’s pieces.
3) Isolated pawns
Can be either a strength, or a weakness. In the endgame it often becomes vulnerable, while in the middlegame it can be quite formidable.
4) Left-behind pawns
While other pawns of the same color have gone a long way, it is still stuck on or near its initial position. It may either be blocked or unable to move since the square in front of it is under attack by the opponent’s pieces.
On a separate note, I would like to elaborate a little on pawn chains. Pawn chains are pawn structures composed of a few connected pawn, e.g. e3-f2-g2-h2. Some pawn chains have pawn bases, i.e. pawns that support the whole structure (e.g. a4-b3-c4-d5 with b3 being the base). In such situations pawn bases act like potential or real weaknesses: destroying the base allows one to eliminate all the other pawns rather easily.
So, one shouldn’t underestimate the importance of pawns. A strong pawn may be worth more than a piece, while a weak pawn may cost you the game. To sum it all up, you should have respect for your pawns (as well as other pieces), but feel the nuances of each particular position.
The following game took place at Aeroflot-2011 with my opponent being the #1 Canadian GM Mark Bluvshtein:
After having played the opening rather carelessly, I had to struggle for equality with White. Mutual mistakes led to an endgame where I made an inaccurate move 27.Qb3 and got myself into trouble. The weak double pawns on the d-file and the left-behind pawn on b2 took their toll.
P.S. Mark has also blogged about his experience at Aeroflot (including the game against me).