After Emanuel Lasker's simplifying defense to the opening in a tournament in 1895, it was out of favor for much of the 20th century, although John Nunn and Jan Timman played some games with it in the late 1970s and early 1980s, and in the 1990s Garry Kasparov used it in a few of his games which prompted a brief revival of interest in it.
The idea behind the move 4.b4 is to give up a pawn in order to secure a strong centre and bear down on Black's weak-point, f7. Ideas based on Ba3, preventing Black from castling, are also often seen. According to Reuben Fine, the Evans poses a challenge for Black since the usual defenses (play ...d6 and/or give back the gambit pawn) are more difficult to pull off than with other gambits.
The most obvious way for Black to meet the gambit is to accept it with 4...Bxb4. After 4.b4 Bxb4 5.c3 the common retreats are:
5...Ba5 According to Chessgames.com, this is Black's most popular retreat. It gets out of the way of White's centre pawns, and pins the c3 pawn if White plays 6.d4, but it has the disadvantage of removing the a5-square for the black king's knight. Black usually subsequently retreats the bishop to b6 to facilitate ...Na5, which is particularly strong when White opts for the Bc4, Qb3 approach.
5...Bc5 According to Chessgames.com, this is the second most popular retreat, with White scoring better than after 5...Ba5. This is often played by people unfamiliar with the Evans Gambit, but is arguably not as good as 5...Ba5, because 6.d4 attacks the bishop and narrows down Black's options as compared with 5...Ba5 6.d4. It is the variation seen in this game.
5...Be7 This is considered the safest retreat and has been played by Anand. CJS Purdy recommended this move if you are unfamiliar with all the nuances of this opening because he believed it was the safest and easiest to play without knowing reams of analysis.After 6.d4 Na5, White can attempt to maintain an initiative with 7.Be2 as played by Kasparov, or immediately recapture the pawn with 7.Nxe5.
5...Bd6 This is called the Stone–Ware Defense after Henry Stone and Preston Ware. The move reinforces the e5-pawn and has been played by several grandmasters such as Andrei Volokitin, Alexander Grischuk and Loek van Wely.
Chessdotcom offers an excellent 5-part series by IM Jacek Stopa on The Evans Gambit HERE.
In the following game Correspondence IM Keith Rodriquez demonstrates that it’s quite playable even in correspondence chess. My personal Correspondence Opening Book which is made up of games played at LSS and ICCF with at least one player rated over 2300 has 65 games with the Evans Gambit. Somewhat surprisingly, White’s score with the Evans is 57.7%. On the other hand, the more ‘solid’ moves 4.c3 only scores 47.4% and 4.d3 scores 45.8%!