The Chiffchaff looks remarkably similar to its close relative, the Willow Warbler, and a little similar to other "Leaf Warblers" such as the Wood Warbler and Dusky, Pallas's, Greenish, Bonelli's and other leaf warblers which are all brown or olive on top with light undersides and a light "supercilium" ("eyebrow stripe").
The key distinguishing feature is their songs and calls, especially the alarm/territorial call,
which is "chiff chaff, chiff chaff" for the Chiffchaff.
There are other subtle differences in appearance.
The key one that I have used here when song has not been heard is that Chiffchaffs have dark legs/feet and Willow Warblers have paler legs/feet
(although that is usually, but not always, the case).
Chiffchaffs nest on the ground in bushes or bramble
Positive Chiffchaff ID by expert (above and below)