Stream cipher algorithms encrypt data one bit at a time. A stream of plaintext bits flows in one side, and a stream of encrypted ciphertext flows out the other. At least, this is the way it works mathematically; in practice, data is always encrypted in byte units.
Stream ciphers are not generally considered as secure as block ciphers, although this will vary depending on the particular algorithm. On the other hand, they do tend to execute faster in software. Ciphertext encrypted with stream ciphers is always the same size as the original plaintext.
Error propagation is usually less when stream ciphers are used. If a bit of ciphertext gets garbled, many stream cipher algorithms will produce only a single bit of garbled plaintext. When a block cipher is used and a ciphertext bit is garbled, at minimum an entire block's worth of plaintext will be garbled.
The only stream cipher provided with the Microsoft Base Cryptographic Provider is the RC4 stream cipher.