A sheva is a vowel in Hebrew (although not a full vowel). It looks like an English colon and always stands beneath a letter as here:
There are three kinds of shevas: video
A vocal sheva which looks like a colon : and is preceded by a long vowel (or is under the first letter of the word).
A silent sheva which also looks like a colon but is preceded by a short vowel.
A composite sheva which is a sheva joined with one of the other vowels.
More here. For practice in distinguishing the two, watch this and this and this.
A sheva is also vocal when:
- it begins a word;
- it is the second of two in a row (when two shevas go walking, the second does the talking);
- it is under a letter with a dagesh-forte in it.
- Gutturals will never take a vocal sheva; it will shift to a composite sheva. They will take a silent sheva.
- Letters with a sheva for the vowel will often lose their dagesh (assuming they had one); see here.
- “A silent sheva is usually placed in a final kaf in order to distinguish the latter from a final nun, e.g. הָלַךְ.” Christo Van der Merwe et al., A Biblical Hebrew Reference Grammar, 38.